The two ways of relating what a person has said are 1)direct and 2)indirect (reported speech).In direct speech, the original speaker's exact words are repeated. In indirect/reported speech, the exact meaning is expressed but not necessarily using the original speaker's exact words. Pronouns and possessive adjectives usually change from first or second person to the third person except when the speaker is reporting his own words. Sometimes a noun can be inserted to avoid doubt.
When we turn direct questions and exclamations into indirect speech, they are converted to affirmative statement .( ? and ! is not used).
Option D is correct as when we convert the given direct question to indirect speech which starts with the question word why , why is used in the indirect speech, to which is used to join the dialogues in the conversation, the appropriate introductory verbs asked and replied are used, the simple present tense is changed to simple past tense, you to he, us to them, and positively is used as Ben agrees to stay with them (of course).
The other options are wrong the simple present tense, said can't be used and of course is not reported.
The correct answer is D)Ann asked Ben why he didn't come and stay with them for a few days to which Ben showed positivity and replied that he would love to come and stay there.