The potential energy of a body is related to the work done by conservative forces on the body. Conservative forces are those that depend only on the position of an object and not on the path taken to reach that position. Examples of conservative forces include gravity and elastic forces.

When a conservative force does positive work on a body, it means that the force is acting in the same direction as the displacement of the body. In other words, the force is aiding the motion of the body.

To understand why the potential energy of the body decreases in this scenario, let's break it down into two parts:

1. Definition of work: Work is defined as the dot product of force and displacement. Mathematically, work (W) is given by W = F * d * cosθ, where F is the magnitude of the force, d is the magnitude of the displacement, and θ is the angle between the force and displacement vectors.

2. Relationship between work and potential energy: The work done by a conservative force is equal to the negative change in potential energy. Mathematically, W = -ΔU, where W is the work, ΔU is the change in potential energy.

Explanation:
When a conservative force does positive work on a body, it means that the force is aiding the motion of the body and the angle between the force and displacement vectors is less than 90 degrees (cosθ is positive). Since work is positive, it implies that the change in potential energy (ΔU) is negative.

According to the relationship between work and potential energy (W = -ΔU), a positive value of work corresponds to a negative change in potential energy. In other words, the potential energy of the body decreases when a conservative force does positive work on it.

Hence, the correct answer is option B) decreases.