Explanation:

When converting direct speech to indirect speech (assertive sentences):

1. Remove the quotation marks and introduce a conjunction like that after the reporting verb (said, told).

2. Change the pronouns according to the subject and object.

3. Adjust the tense if the reporting verb is in the past:

   • Modal verbs like must generally change to had to in the past tense, unless expressing eternal truths.

Example:

• Direct: Kalim said, "I must go there."

• Indirect: Kalim said that he had to go there.

Other options are wrong because:

• ক) “I” is not changed to “he” → incorrect pronoun

• খ) “must” is unchanged → past context usually requires had to

• ঘ) “must had to” → grammatically incorrect; two modal verbs cannot be used together

Source: A Passage to the English Language, S.M. Zakir Hussain

• The incorrect synonym pair is — খ) Intrepid: Unheroic

• Intrepid (অকুতোভয়; নিঃশঙ্ক; অসমসাহসিক) এবং Unheroic (অবীরোচিত; ভীরুতার পরিচায়ক) পরস্পর বিপরীত অর্থের, সমার্থক নয়।

• Other options:

• ক) Extirpate (সম্পূর্ণ ধ্বংস করা) ↔ Destroy (ধ্বংস/নষ্ট/বিধ্বস্ত/বরবাদ করা)

• গ) Amenable ((ব্যক্তি) সংবেদনশীল; বাধ্য; (আইন সম্বন্ধীয়) (ব্যক্তি) দায়ী) ↔ Responsible ((ব্যক্তি) আইনত অথবা নীতিগতভাবে দায়ী)

• ঘ) Temerity (হঠকারিতা; দুঃসাহস) ↔ Audacity (দুঃসাহস; স্পর্ধা; ধৃষ্টতা)

• এই শব্দগুলো পরস্পর সমার্থক (synonyms)।

Source:

1. Accessible Dictionary, Bangla Academy

2. Merriam-Webster Dictionary

3rd Conditional Rules:

• If-clause: had + past participle (V3)

• Main clause: would/could/might + have + past participle (V3)

Example from your sentence:

• If they had listened to the advice, they might have avoided the problem. 

Explanation:

• "had listened" → If-clause in past perfect

• "might have avoided" → Main clause showing hypothetical result in the past

Other options would not follow the 3rd conditional structure, so they are incorrect.