The answer here is C. The bank in this case would be able to lend out $90 million.
The reserve requirement for a bank is the percentage of its deposits that it must keep on hand. The rest of the money can be lent out. Banks want to lend so that they can make money. However, they must also keep reserves so they do not fail if they have some loans go bad.
When we say, then, that the reserve requirement is 10% that means that the bank must keep 10% of its deposits and it may lend out 90%. 90% of $100,000 is $90,000. Therefore, C is the right answer.