The correct answer is **was** (Option 4). **Comprehensive Derivation:** The correct answer is 'was'. Explanation & Key Takeaways: • 'although' is used to introduce a subordinating clause which makes the main clause seem surprising. • We use Simple past tense to talk about something that has already happened. • Example: Although he came late, he was allowed to attend the class. • Out of all the given options, the most suitable modal verb is 'was'. • Hence, the correct answer is option 4. Correct Sentence: Although he didn't have a ticket, Ken was allowed to come in. Related Concepts: • could be: used to express possibility, past ability, suggestions • can be: used to show ability, granting permission etc. • might: to express possibility **Option Elimination:** • **Option 1 (could be)**: Incorrect distractor. • **Option 2 (can be)**: Incorrect distractor. • **Option 3 (might)**: Incorrect distractor. • **Option 4 (was)**: Correct Answer; accurate according to canonical reference. **Exam Trap:** Be careful of misleading distractors and subtle terminology differences. Verify core properties before selecting your answer. **Memory Booster:** Core mnemonic rule: was corresponds to Option (4).