Select the correct meaning of the underlined phrase or idiom.
He did me a good turn by recommending me for the post of Vice- Principal
 

We know the meaning and therefore the origin of the given idiom. Now, let’s have a look at its usage:
Example- My friend did me a decent turn by helping me with my project. Here, we are able to understand that X’s friend helped him along with his project.
In the given question- X helped Y by recommending him for the post of Vice-Principal.
Let’s examine the given options-
Became suddenly good- It refers to a sudden change in an exceedingly person’s behavior. it's different in assuming to the above-given idiom. Hence, it's an incorrect option.
Improved my prospects- It means to enhance one’s views and outlook in respect to the long run. it's different in assuming to the given idiom. Hence, it's an incorrect option.
Did an act of kindness- It means to assist someone. it's similar in assuming to the given idiom. Hence, it's the proper option.
Returned my kindness- It means to return someone’s favor. it's different in aspiring to the given idiom. Hence, it's an incorrect option.
So, the proper answer is “Option c”.