To calculate the free surface correction, it is necessary to divide the free-surface moments by the __________.

Correct Answer: Option D — total displacement

Moment of inertia varies proportionally with the cube of the tank's breadth. Installing a centerline bulkhead halves the breadth, reducing the moment of inertia to one-fourth ($1/2^3 = 1/8$ for two tanks, scaling down correctly by a factor of 4).