Option C.
Step-by-step explanation:
Let the total number of men in the library is x and total number of women is y.
As per statement, " ratio of men to women is 3 : 1"
That means
or x = 3y
Now we add 3x on both the sides of the equation
x + 3x = 3y + 3x
4x = 3(x + y)
x =
=
Therefore, of the members are men.
Option C will be the answer.