Question

Q.No.1: A subset of {1,7,8 } is:

Answer 1

Explanation:

you only need to provide an example ?

if your question here is complete, then the full set is

{1,7,8}

and these are all possible subsets (pick any one you like) :

{1}, {7}, {8}, {1,7}, {1,8}, {7,8}

and formally also

{}, {1,7,8}

the empty set is a subset of every set, and the whole set itself is also formally a subset.