(c) $21 \%$ of what is $10.5 ?$

Step 1 ：We need to find a number such that 21% of it equals 10.5. This can be represented as an equation: \(0.21 * x = 10.5\), where x is the number we're looking for.

Step 2 ：We can solve this equation for x by dividing both sides by 0.21.

Step 3 ：\(x = \frac{10.5}{0.21}\)

Step 4 ：\(x = 50.0\)

Step 5 ：Final Answer: \(21 \%\) of \(50\) is \(10.5\). So, the answer is \(\boxed{50}\).