You are absolutely right man.....The riddle is so simple that you don't need too much thinking to get it right...
The answer lies in the riddle and from this riddle you can observe that letter 'm' occurs once in the word 'minute', twice in 'moment' and none in ' one thousand years'......
So in conclusion, the answer is " The letter m"... Keep the great work going on....HACKGH FOR LIFE......