a*a=a, a*b=b, a*c=c, a*d=d
Clearly, from this first row of the table, a is an identity element, and identity element can't be a generator
Eg: Suppose we have set-> {1,2,3} and operation is multiplication modulo 4, so here identity element is 1, now see:
(1^1)mod 4=(1) mod 4=1
(1^2)mod 4=(1*1) mod 4=1
(1^3)mod 4=(1*1*1) mod 4=1
So, identity element keep on repeating the same number again & again, hence it can't be a generator.
So, options A and D are ruled out.
Now, from option B and C, c is common in both, so definitely, c is a generator, and now go to that row which contains c, there, c*d=a, which means, d is inverse of c, or c and d are inverses of each other and there is a property that, if an element is a generator then its inverse is also a generator, hence d is also a generator.
So, we are left with option C only.