A AB ABC ABCD ABCDE.

#include <stdio.h>

int main(void)
{
int i, j;
char ch;

i = 1;
while(i <= 5)
{
j = 1;
ch = 'A';
printf("\n");
while(j <= i)
{
printf("%c", ch);
j++;
ch++;
}
i++;
}

return 0;
}