I'll write ~x for .

a) is correct if you stop at the second last line, which is the simplest expression. Is the last line (y + ~z)(y + ~z)? Then it is incorrect because it does not have x.

In b) you did not change disjunction into conjunction going from line 1 to line 2. Also, x + 1 + ~z = 1, not 0.

In c), you did not copy a + between z and the term in square brackets. Also, what law did you use to go from line 1 to line 2?