The correct answer is C.
A good turn is an act of kindness or generosity. In this case, the person recommended the speaker for the post of Vice-Principal, which is an act of kindness that could improve the speaker’s prospects.
Option A is incorrect because it does not describe an act of kindness. Option B is incorrect because it does not describe an act of kindness that could improve the speaker’s prospects. Option D is incorrect because it does not describe an act of kindness that the person did for the speaker.