The correct answer is - ক) Though he was ill, he attended the meeting.
এটি একটি complex sentence যাতে একটি মূল clause ("he attended the meeting") এবং একটি subordinate clause ("though he was ill") রয়েছে।
"Though he was ill" সঠিকভাবে "in spite of his illness" এর অর্থ প্রকাশ করে, কারণ এটি অসুস্থতার কারণ বোঝায়।

Simple sentence-এ যদি In spite of ব্যবহৃত হয়, তাহলে Complex sentence করতে হলে Though/Although ব্যবহার করতে হয়।
আবার, Complex sentence-এ Though/Although থাকলে, সেটিকে Simple sentence করতে হলে In spite of বসে।
• Example:
Simple: In spite of his poverty, he is happy.
Complex: Though he is poor, he is happy.

• More examples:
Simple: In spite of the bad weather, we went there.
Complex: Though the weather was bad, we went there.

Simple: In spite of his bad behaviour, people mix with him.
Complex: Though his behaviour is bad, people mix with him.

Simple: In spite of his honesty, he is not happy.
Complex:Though he is honest, he is not happy.
or, Honest as he is, he is not happy.

Simple: In spite of his cruelty, I love him.
Complex: Though he is cruel, I love him.

Other options
খ) Though he is illness, he attended the meeting
এটি ব্যাকরণগতভাবে ভুল। "Illness" একটি noun, কিন্তু "though" এর পরে একটি সম্পূর্ণ clause প্রয়োজন (subject + verb)।
এখানে "he is illness" অর্থহীন কারণ "is" এর পরে adjective ("ill") প্রয়োজন, noun নয়।

গ) Though he illness, he attended the meeting
এটিও ব্যাকরণগতভাবে ভুল।
"He illness" একটি সম্পূর্ণ clause নয়, কারণ এতে কোনো verb নেই।
"Though" এর পরে একটি পূর্ণাঙ্গ clause প্রয়োজন।

ঘ) Though his illness, he attended the meeting.
এটি ভুল কারণ "his illness" একটি noun phrase, কিন্তু "though" এর পরে একটি সম্পূর্ণ subordinate clause (subject + verb) প্রয়োজন।
"His illness" কোনো clause নয়।

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