The correct option is B she remembered the horrors of war
A complex sentence is made up of two or more clauses, where at least one
of them should be independent, i.e., complete in meaning. The other
clause/clauses should be dependent; it should depend on the
main/independent clause for its complete meaning. In the given sentence, there are four clauses, one of which is the main clause as it is not dependent on the others for its complete meaning. The main clause is "she remembered the horrors of war" as it is complete as a sentence on its own, hence option B is correct. The clauses in options A and D are subordinate/dependent, they both begin with a time-related conjunction ("Whenever" and "while" respectively) and sound incomplete on their own. Hence they are not main clauses and they're incorrect. Option C contains a 'that' clause which forms a clause of its own with the subject and verb, but it cannot stand alone as a sentence, hence it is not the main clause and C is incorrect.