The force of gravity is equal to the mass times the centripetal acceleration:
Fg = m v² / r
Also, the force of gravity is defined by Newton's law of universal gravitation, which states the Fg = mMG / r², where m and M are the masses of the objects, G is the universal constant of gravitation, and r is the distance between the objects.
mMG / r² = m v²/ r
MG / r = v²
This means the square of the orbital velocity is equal to the mass of the sun times the universal constant of gravity divided by the orbital radius. So whichever planet has the smallest orbital radius will have the highest orbital velocity. Of the four options, that would be Mercury.
