The correct answer is abundance. The sentence suggests that Kit has more zucchini than she needs, and is therefore trying to offload zucchini on anyone who might want some. The plethora might lead to a mild irritation, but the words are definitely not synonyms. (In some cases, a plethora is certainly not an irritation.) The word plethora is related to a quantity, but is a specific type of quantity; an excess. Because the word quantity can also describe a lack of something, these words are not synonyms. Kit is obviously trying to avoid waste, but the words plethora and waste are not synonyms. Waste would also not be a natural replacement for plethora in the sentence.