About part b:

Since g is a primitive root mod p, the order of g (mod p) must be p-1, and we have =1(mod p)

In part b, we have to show that if gcd(k,p-1)=d>1, then does NOT form a reduced residue system mod p. If we can show that some distinct elements in the set are congruent to each other, then we're done. But how to prove show this???

Can someone kindly help me, please? I'm still puzzled...