A. False. Consider the identity matrix, which is diagonalizable (it's already diagonal) but all its eigenvalues are the same (1).
b. True. Suppose

is the matrix of the eigenvectors of

, and

is the diagonal matrix of the eigenvalues of

:


Then

In other words, the columns of

are

, which are identically

, and these are the columns of

.
c. False. A counterexample is the matrix

which is nonsingular, but it has only one eigenvalue.
d. False. Consider the matrix

with eigenvalue

and eigenvector

, where

. But the matrix can't be diagonalized.