23) E

It is important that you understand that the artery is in a shape of a cylinder. 

The cross-sectional area would be the area of the circle in the cylinder: 

Let us first list the information that we are given: 

Length of artery = x

Speed of cell = V

Time taken by cell to travel length of artery = T

Let cross sectional area = A

Volume of blood flowing through the artery in one second = (Ax ÷ T)

So A = (VT ÷ x)

The units for this would be ml/mm. We know 1 ml = 1 mm³

So A = (VT ÷ x) x 10³

Hence, E is the answer.