The correct answer is (a) 23.5%.
Let $A$ be the third number. Then $B$ is 15% less than $A$, so $B = 0.85A$. $C$ is 32% less than $A$, so $C = 0.68A$.
To find the percentage by which $C$ is less than $B$, we can use the following formula:
Percentage change = $\frac{new – old}{old} \times 100\%$
In this case, the new value is $C$ and the old value is $B$. So, the percentage change is:
Percentage change = $\frac{0.68A – 0.85A}{0.85A} \times 100\% = 23.5\%$
Therefore, $C$ is 23.5% less than $B$.
Here is a brief explanation of each option:
(a) 23.5%: This is the correct answer. It is calculated by using the formula above.
(b) 22%: This is incorrect. It is calculated by using the formula above, but with the wrong values.
(c) 20%: This is incorrect. It is calculated by using the formula above, but with the wrong values.
(d) 18%: This is incorrect. It is calculated by using the formula above, but with the wrong values.