$\begin{bmatrix} (0,0)& (0,1)& (0,2)& (0,3)\\ (1,0)& (1,1)& (1,2)& (1,3)\\ (2,0)& (2,1)& (2,2)& (2,3)\\ (3,0)& (3,1)& (3,2)& (3,3) \end{bmatrix}$
1D array for lower triangular matrix : $\begin{bmatrix} (0,0) & (1,0) & (1,1) & (2,0) & (2,1) & (2,2) & (3,0) & (3,1) & (3,2) &(3,3) \end{bmatrix}$
Now check for options:
index=3 i.e. (2,0) doesn't satisfy option A (index=i+j)
index=0 i.e. (0,0) doesn't satisfy option B (index=i+j-1)
index=0 i.e. (0,0) doesn't satisfy option c (index=ij-1+i(i-1)/2)
index=2 i.e. (1,1) doesn't satisfy option B (index=i+j(j-1)/2)
i think no options are correct