The sentence which has a relative pronoun is-

Relative Pronoun:
Who, whom, which, that, whose ইত্যাদি। এর পূর্বে সবসময় ‍antecedent থাকতে হবে।
কোন বাক্যে Relative Pronoun থাকলে তা নিঃসন্দেহে complex sentence.
উৎসঃ Advanced Learner's Grammar and Composition by Chowdhury and Hossain & A passage to the English Language- S.M. Jakir Hussain & English Grammar in Use by Raymond Murphy