D. I don't know which is irritating me more right now: the ringing phone, the crying baby, or the dog that won't stop begging at my feet.
Sentence D does not use parallel structure. When we have a list we want to make sure we use parallel structure. In other words, we want to make sure we use the same verb tense throughout our list (present tense, past tense, present progressive tense (-ing), etc...) Sentence D does not. The verbs are ringing, crying, and won't stop begging. For it to be correct, we would have to change the last part to the begging dog.