Ans d is correct.
set of all integer is countable. Generate like 1, -1,2,-2... so for a number x. I will take 2x steps to generate it or the corresponding natural number will be 2x. And set of all strings is also countable .
Because (a) is incorrect due to there exist a one to one correspondence between natural no and sigma *
(b) is also incorrect because sigma* multiply as much time it remain same
(c) power set of every countable infinite set is always uncountable set
so d is correct option