Answer is B instead of infinity just take 99 then i can write 1/n(n+1) as 1/n-1/n+1 if you write like this middle terms will cancel only first and last term will remain 1-1/100 so 99/100 so 0.99 similarly if you take 999 sum wil be 0.999 so on for infinity it wil be 0.99999.....=1 so 1 is the answer.