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.