Which law does this diagram show?

The diagram you are referring to likely illustrates **Kepler's Second Law**, also known as the **Law of Equal Areas**. This law states that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time, meaning that a planet moves faster when it is closer to the Sun and slower when it is farther away.

Therefore, the correct answer is:

**a. Kepler's second Law: Law of Equal Areas**.