C++ Program:

Example:

A
A B
A B C

#include <iostream>
using namespace std;

int main() {
  int rows;
  char c;
  cout<<"Enter number of rows : ";
  cin>>rows;
  for(int i=1; i<=rows; i++) {
    c='A';
    for(int j=1; j<=i; j++) {
      cout<<c++<<" ";
    }
    cout<<endl;
  }
  return 0;
}

Output:

  Enter number of rows : 4
  A
  A B
  A B C
  A B C D

Main logic :

for(int i=1; i<=rows; i++) {  //rows : number of rows to print
  c='A';
  for(int j=1; j<=i; j++) {
    cout<<c++<<" ";
  }
  cout<<endl;
}

You can print the same pattern in small alphabets, just set c='a'