Python 2, (削除) 63 (削除ここまで) 61 bytes

-2 bytes thanks to Rod.

i=0
exec"print' '*i+'%c '%(i+65)*-~i;i+=1;"*(ord(input())-64)

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Python 2, (削除) 63 (削除ここまで) (削除) 61 (削除ここまで) 59 bytes

-2 bytes thanks to Rod. -2 bytes thanks to Felipe Nardi Batista.

i=1
exec"print' '*i+'%c '%(i+64)*i;i+=1;"*(ord(input())-64)

Try it online!

Python 2, 63 bytes

i=0
exec"print' '*i+('%c '%(i+65))*-~i;i+=1;"*(ord(input())-64)

Try it online!

Python 2, (削除) 63 (削除ここまで) 61 bytes

-2 bytes thanks to Rod.

i=0
exec"print' '*i+'%c '%(i+65)*-~i;i+=1;"*(ord(input())-64)

Try it online!