Question

Which of the following is not an abstract noun?
A. Truth
B. Sky
C. Beauty
D. Faith

Answer 1

The noun Sky is not an abstract noun. 
Sky is a concrete noun. A concrete noun is a word for something that can be seen, heard, felt, tasted or smelled. 

Answer 2

An abstract noun is a noun form in which the material form is not tangible, it is not in the form of a substance.

Therefore Truth is abstract, so is Beauty and Faith.

But Sky is not an abstract noun.  It has a material form.

So option B is the correct one.