The correct option is D drops
Tense is used to provide more information about an action ( its time, completeness, etc.). Option A)drops is correct as the principal cause uses the simple present tense and therefore, the subordinate clause can use simple present, simple past, simple future or present perfect tense and as the sentence talks about a general fact, simple present tense must be used. The other options are wrong as no other tense is appropriate for the given sentence. The correct answer is A)drops.