Answer 1

Simplest method:
Allocate a square matrix.
char matrix[5][5];

Fill it with stars:
for (int line = 0; line < 5; line++)
for (int colum = 0; column < 5; column++)
matrix[line][column] = '*';

Remove a line:
for (int line = 0, column = 2; line < 3; line++, column--)
matrix[line][column] = ' ';

Repeat for the other three lines of your diamond.