The kinetic energy of a planet in orbit varies according to its distance from the sun due to the conservation of angular momentum. The closer it is to the sun, the faster it moves and thus the greater its kinetic energy. Conversely, the farther it is from the sun, the slower it moves and the less its kinetic energy.

Option 1 (A) is correct because this point represents where Mercury is closest to the sun, which is known as perihelion. It is at this point that Mercury moves the fastest and thus has the highest kinetic energy.

Option 2 (B) and Option 4(D) represents points farther from the sun and hence Mercury will be moving slower here with less kinetic energy.

Option 3 (C) is the furthest point from the sun, known as aphelion, and here Mercury would have its lowest kinetic energy.

Therefore, Option 1 (A) is the correct answer as the kinetic energy of Mercury is greatest at this point.