Which sentence shows the most effective dialogue? john, you are ill; we have to get you to the good samaritan hospital to save you. john, how ill you appear! what shall we do? john, you look sick. what's wrong? john, as ill with the flu as you are, driving to mexico seems highly unreasonable.

C: John, you look sick. What's wrong?

Explanation:

 While all the other sentences only manifest opinions, suggestions, and decisons already made concerning John's condition and care, even some despair, the sentence in C, besides mentioning his appearance, also allows John to express what or how he is feeling by asking  'what's wrong?', which is a form of dialogue, and the inclusion of someone's opinion.