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(man) Did you hear that my parents are planning a trip to Vancouver?
(woman) What for?
so What does the woman want to know? thank you for answers
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"what for" = "for what purpose" = "why?"

"WHAT did you do that FOR?" = "WHY did you do that?"
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"Why" is different from "what for".

WHY --> reason or motive.
WHAT FOR --> purpose.

A: "Why are you here?"
B: "Because my father called me."
A: "What for?" (= What did he call you for?)
B: "I don't know; he only told me to come.

"A" knows the reason why they are here (wherever that is), but they don't know what the purpose of their being there is.

Miriam
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Comments  
here is the four choices to this question,but i dont think there is an answer(A) What travel plans Philip is making
How Philip s parents are going to travel
[C]What Philip said
[D]Where Philip heard the news
Native speaker's opinion.

Sorry Lawthinker, but I can't think of any situations where one of your meanings would apply, not in British English anyway.

The only meaning for 'What For' is to ask why.

She wants to know what they are going to Vermont for - i.e. why? Are they visiting relatives, looking for a new home, going on a holiday?

A little boy hits his brother over the head. the mother asks 'What did you do that for? i.e. Why?

'What for' is a shortening of a whole question asking why.

Mother: Get down from the tree!
Kid: 'What for' would be a cheeky way of saying No!

And the reply form parents is usually 'I'll give you what for!' delivered in a (usually mock) threatening tone implying physical punishment.
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 miriam's reply was promoted to an answer.