Concept: This sentence is a conditional sentence, specifically a Type 3 Conditional (also known as the third conditional or past unreal conditional). It describes a hypothetical situation in the past that did not happen, and its hypothetical past result.
Step 1: Structure of a Type 3 Conditional Sentence A Type 3 conditional sentence has two clauses: % Option (A) The 'if' clause (condition): Describes the unreal past condition. Structure: If + Past Perfect tense (had + past participle) Example: "If I {had worked} hard..." % Option (B) The main clause (result): Describes the unreal past result. Structure: Subject + would have + past participle (Other modal verbs like "could have" or "might have" can also be used depending on the nuance).
Step 2: Apply the structure to the given sentence The 'if' clause is "If I had worked hard," which is in the Past Perfect tense ({had worked}). This confirms it's a Type 3 conditional. Therefore, the main clause should use "would have + past participle" (or "could have / might have + past participle"). The verb is "score," and its past participle is "scored." So, the main clause should be "I would have scored very high marks..."
Step 3: Evaluating the options
(1) would have scored: Correct. This fits the structure of the main clause in a Type 3 conditional.
(2) scored (Simple Past): Incorrect. This doesn't fit the unreal past conditional structure.
(3) would score (would + base verb): Incorrect. This is used in Type 2 conditionals (unreal present/future).
(4) could score (could + base verb): Incorrect. This is used in Type 2 conditionals. For Type 3, it would be "could have scored." The correct completion is "would have scored."