Problem 121

Since 99 is close to 100 and 9 is close to 10, the value of $99\times9$ is approximately equal to $100\times10=1000$.

Of the given answers, this means that the integer that is closest to $99\times9$ is most likely 1000.

Multiplying, we get that the value of $99\times9$ is 891.
Of the given answers, $99\times9$ is indeed closest to 1000.