Answer:

It is already a complex SENTENCE. A complex sentence is defined as a sentence MADE up of one or more independent clauses plus one or more dependent clauses. An independent clause consists of a group of words containing a subject and a predicate and which could stand alone as a sentence. A dependent clause consists of a group of words CONTAING a subject and a predicate but which could not stand alone as a sentence.

In the sentence you have presented, Bangladesh can engage her HUMAN resources is an independent clause. after she has explored all other resources is a dependent clause.

Bangladesh can engage her human resources has a subject, Bangladesh, and a predicate, can engage, and with a period (full stop) after the WORD "resources" can stand alone as a sentence. Thus this is an independent clause.

after she has explored all other resources has a subject, she and a predicate, has explored, but even with a capital letter and a period cannot stand alone as a sentence. Thus this is a dependent clause.

An independent clause plus a dependent clause make a complex sentence.