Selecting Primes from list of list

You may try this:

Select[PrimeQ] /@ list

You can also use Pick with PrimeQ @ list as the selector array:

Pick[#, PrimeQ @ #] & @ list

{{2, 3, 5, 7}, {11}, {17}}

or Cases:

Cases[_?PrimeQ] /@ list

{{2, 3, 5, 7}, {11}, {17}}