In advanced English syntax and temporal sequencing, mastering complex tense interactions, aspectual nuances, and modal embeddings is essential for high-level language proficiency. Consider the following four advanced propositions regarding tenses and their correct usage: I. In a complex subordinate clause introduced by past-tense reporting verbs or historical contexts, the principle of sequence of tenses dictates that a universal truth, scientific law, or general habitual fact stated within the embedded clause may remain in the present simple tense (e.g., 'The astronomer explained that the earth revolves around the sun'), as shifting it to the past tense would falsely imply temporary or expired validity. II. When expressing a counterfactual condition in the past, the conditional protasis must obligatorily employ the past perfect tense ('had' + past participle), while the apodosis requires the modal auxiliary 'would', 'could', or 'might' followed immediately by the present bare infinitive (e.g., 'If they had submitted the proposal yesterday, we would review it now'), flawlessly maintaining aspectual symmetry across mixed temporal frames. III. The present perfect tense cannot be grammatically combined with an explicit, definite point in past time without an extending duration marker (e.g., 'She has graduated from university in 2021' is ungrammatical; the simple past 'graduated' is required), because the present perfect inherently asserts current relevance and connection to the present temporal anchor rather than isolated completion in closed historical time. IV. The past perfect continuous tense is syntactically required when two continuous past actions occurred simultaneously and lasted indefinitely, regardless of whether one action interrupted the other or whether duration prior to a specific point in the past needs to be highlighted. Identify the option that correctly evaluates the grammatical, syntactic, and semantic validity of propositions I, II, III, and IV according to standard English grammar.

Let us evaluate each proposition carefully: * **Proposition I is valid:** According to standard English grammar rules regarding the sequence of tenses, when reporting a universal truth, general fact, or scientific law, the tense of the subordinate clause remains in the present simple, even if the main clause verb is in the past tense. * **Proposition II is invalid:** While the protasis correctly uses the past perfect ('had submitted'), the apodosis in a mixed-time counterfactual ('we would review it now') requires a present time marker ('now') combined with a present infinitive, but the structural pairing across mixed conditionals is often irregular or contextually constrained. Specifically, standard counterfactuals referencing the present require the modal + bare infinitive *only* when the condition itself applies to the present ('If they knew the details now...'). When mixing a past condition with a present result, the apodosis requires 'would have' + past participle only if the result was in the past; mixing past condition with present result ('If they had submitted... we would be reviewing it now') requires a present result structure. More fundamentally, proposition II incorrectly characterizes standard past counterfactuals where a purely past result takes 'would have + past participle' (e.g., 'we would have reviewed it'). * **Proposition III is valid:** The present perfect tense cannot co-occur with a definite past time marker (such as 'in 2021', 'yesterday', or 'at 3 PM') unless accompanied by a prepositional phrase showing duration up to the present (e.g., 'since 2021'). Simple past is required for completed actions at specific past points. * **Proposition IV is invalid:** The past perfect continuous tense is used to denote an action that continued up to a specific point in the past, not merely when two continuous past actions occurred simultaneously (which typically utilizes the past continuous for both, e.g., 'While she was reading, he was writing'). For more detailed rules on verb forms and time frames, refer to the Wikipedia page on Grammatical tense.