The correct option is A To join two sentences.
We have to choose the option that does not describe the function of a sentence.
Interrogative sentences ask a question, 'What are you doing right now?'
Exclamatory sentences exclaim or express sudden emotions like "Hurrah! We won.
Declarative or Assertive sentences declare or state something, 'I received a job today.'
Hence these are the functions of a sentence, and B, C and D are incorrect.
We choose option A because this is not the function of a sentence. Conjunctions join two or more sentences, a sentence itself cannot do that.