Question

which of the flowing are the subspace of Mn*n
(a) the set of all diagonal n*n matrices.
(b)the set of all n*n matrices A such that det(A)=0
(c)the set of all n*n matrices a such that tr(A)=0
(d)the set of all symmetric n*n matrices.
(e)the set of all n*m matrices A such that A^t=-A.
(f)the set of all n*n matrices A for which Ax=0 has only the trivial solution.
(g) the set of all n*n matrices A such that AB=BA for some fixed n*n matrix B