A sentence/a part of the sentence is underlined. Four alternatives are given to the underlined part which will improve the sentence. Choose the correct alternative and click the button corresponding to it. In case no improvement is needed, click the button corresponding to "No improvement".
An old friend, may I give you an advice?

Advice is not countable. Not "an advice" but "advice" or " some advice". 

The word advice is an uncountable noun. Uncountable nouns do not have plural forms. They cannot be used with the articles a/an.

He gave me some advice. (NOT He gave me an advice.) (NOT He gave me advices.) Incorrect: Let me give you an advice.  Correct: Let me give you some advice. Incorrect: She was always there to give great advices. Correct: She was always there to give great advice.

Hence option A