Step 1: Identify the logical relationship. Ravi studied hard for six months. Yet he performed poorly. This is a CONTRAST — the expected result (doing well) did not happen despite the effort. Step 2: Test each option for logic. - Although: introduces a contrasting clause — 'although X, Y' means Y is surprising given X. This fits perfectly. - Because: signals that X caused Y. Studying hard does NOT cause poor performance — wrong logic. - Since: read as causal here ('since he studied hard, he performed poorly') — inverts the intended meaning. Step 3: Test 'Despite' for grammar. 'Despite' CANNOT introduce a full clause (subject + verb). It needs a noun or gerund: - WRONG: Despite Ravi had studied hard... - CORRECT: Despite having studied hard, Ravi... So 'Despite' fails on a grammar rule, not just logic. Step 4: Confirm 'Although'. 'Although Ravi had studied hard for six months, he could not understand...' — logically and grammatically correct. Answer: Although