..

Male (born at zoo) + Male (not born at zoo)
= 75% of 288 = 216
Female (born at zoo) + Female (not born at zoo)
= 25% of 288 = 72
Question : What % of the mammal were born at zoo?
Statement 1 : The number of male mammals that were born at zoo is 3 times the number of female mammals that were not born at zoo.
Therefore, Male (born at zoo) = 3x female (not born at zoo)
The information here is not enough to determine the % of mammals born at zoo.
Hence, statement 1 is NOT SUFFICIENT.

Statement 2 : The number of male mammals that weren't born at zoo is 3 times the number of male mammals that were born at zoo.
This means it's in ratio 3 : 1
$$Male\left(born\ at\ zoo\right)\ =\ \frac{216}{3\ +\ 1}\ =\ \frac{216}{4}\ =\ 54$$
but there i no information about the female, hence, statement 2 is INSUFFICIENT.
Statement 1 and 2 together
Male (born at zoo) = 3 * female (not born in zoo)
Male (born at zoo) = 54
$$\frac{54}{3}=\ \frac{\left(3\ \cdot\ female\ \left(not\ born\ at\ zoo\right)\right)}{3}$$
Female (not born at zoo) = Total no. of females (not born in zoo)
= 72 - 18 = 54
Mammals (born at zoo) = Male (born at zoo) + Female(born at zoo)
Mammals (born at zoo) = 54 + 54 = 108.
$$\%\ of\ mammals\ born\ at\ zoo\ =\ \frac{108}{288}\cdot\ \frac{100}{1}$$
= 37.5%
Statement 1 and 2 combined together is SUFFICIENT.
Option C is CORRECT.