Mindful of the growing popularity of backyard compost piles among home gardeners, (experts warn that adding meat, diary products, or cooking grease to compost will) attract vermin. (A) as it is (b) adding meat, diary products, or cooking to compost, this is what experts warn will (C) and with warnings from experts that adding meat, -X- (D)warning from experts concerning the addition of -x- (e) experts warning that adding meat,-X- would. Why A?
This is a dangling modifier. The beginning of the sentence describes a group of people who are mindful of a thing. After that first comma, that group of people must be named! Only a choice that begins with “experts,” therefore, is acceptable. Only two choices begin with “experts”—A and E. Because E isn’t a complete sentence (it doesn’t have a main verb with “warning” instead of “warn”), you know it’s A.