সঠিক উত্তর হলো: ক) That she will reciprocate my passion for her is certain.

এটি একটি complex sentence, কারণ এতে -
- Main clause: is certain.
- Subordinate (noun) clause: 'That she will reciprocate my passion for her',
- এখানে “that” দিয়ে শুরু হওয়া noun clause-টি পুরো বাক্যের subject হিসেবে কাজ করছে।

• Complex sentence:
- যে sentence এ একটি Principal clause এবং এক বা একাধিক Subordinate clause থাকে, তাকে Complex sentence বলে।
- Complex sentence এর subordinate clause এর শুরুতে সাধারণত if, though, although, as, because, since, so that, that, until, till, unless, when, why, who, which, where, how, before, after, whether, while বসে।

Other options,

খ) He did his work and I, mine.
- Compound Sentence.
- দুটো independent clause: "He did his work" এবং "I [did], mine"
- "and" দিয়ে যুক্ত (coordinating conjunction) → দুটো clause সমান গুরুত্বপূর্ণ। এটা compound.

গ) In case of your failure to move, you will die.
- এখানে "In case of your failure to move" = prepositional phrase (একটি clause নয়, শুধু phrase)।
- পুরো বাক্যে শুধু একটি independent clause ("you will die") আছে।
- এটি simple sentence (শুধু একটি independent clause + phrase)।

ঘ) Because of his playing well, he won the game.
- "Because of his playing well" = prepositional phrase (gerund phrase), clause নয়।
- শুধু একটি independent clause ("he won the game").
- এটিও simple sentence