What does this phrase mean: "to ask her out"?

[Martini]

English Language Tests, Intermediate level

ESL/EFL Test #165 "Phrasal Verbs (A to B)", question 3

Ken really likes Kirstie but he's too nervous to ..........

(a) ask her out
(b) ask her on
(c) ask out

English Language Tests, Intermediate level

ESL/EFL Test #165 "Phrasal Verbs (A to B)", answer 3

Ken really likes Kirstie but he's too nervous to ask her out.

Correct answer: (a) ask her out

Your answer was: incorrect
Ken really likes Kirstie but he's too nervous to ask her on.
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Dear Teacher, can you explain the phrase verb "ask her on"?

Thank you
Martini

[Diverhank]

I don't think there's such a phrase as "ask her on". "Ask her out" means to ask her to go out on a date.