If N is 4 and m is the matrix displayed below, which code fragment below stores 1's along m's main diagonal and leaves all other entries unchanged?

```
m 0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
```

a. for ( int i = 1; i < N; ++i )
for ( int j = i; j <= N; ++j )
m[i][j] = 1;

b. for ( int i = 1; i < N; ++i )
m[i][i] = 1;
c. for ( int i = 0; i < N; ++i )
for ( int j = i; j <= N; ++j )
m[i][j] = 1;

d. for ( int i = 0; i < N; ++i )
m[i][i] = 1;

d. for ( int i = 0; i < N; ++i )
m[i][i] = 1;