Null Matrix

Q. Which of the following is a scalar matrix

1. $\left[\begin{array}{ccc}5& 0& 0\\ 0& -5& 0\\ 0& 0& 5\end{array}\right]$
2. $\left[\begin{array}{ccc}6& 0& 0\\ 0& 6& 0\\ 0& 0& 6\end{array}\right]$
3. $\left[\begin{array}{ccc}\frac{1}{2}& 0& 0\\ 0& 1\frac{1}{2}& 0\\ 0& 0& 2\frac{1}{2}\end{array}\right]$
4. $\left[\begin{array}{ccc}5& 0& 0\\ 0& 6& 0\\ 0& 0& 7\end{array}\right]$

Q.

Let $\omega$ be a complex cube root of unity with $\omega \ne 0$ and $P=\left[p_{ij}\right]$ be an $n\times n$ matrix with $P_{ij}={\omega }^{i+j}$. Then $P^2=0$ when n is equal to

1. 57

2. 55

3. 58

4. 56