**Explanation:**

The correct answer is option D, a prismatic compass.

A prismatic compass is a type of compass that is commonly used in surveying and navigation. It consists of a magnetic needle, a graduated circle, and a prism. The prism allows the user to read the compass bearing directly through the prism without having to look directly at the compass needle.

**Components of a prismatic compass:**

1. **Magnetic needle:** The compass needle is a small magnet that aligns itself with the Earth's magnetic field to indicate the magnetic north. It is mounted on a pivot so that it can rotate freely.

2. **Graduated circle:** The graduated circle is marked with degrees and is used to measure the bearing or direction. It is fixed and does not move.

3. **Prism:** The prism is a transparent glass or plastic prism that is attached to the compass. It allows the user to see both the compass needle and the graduated circle at the same time. By aligning the prism with the desired target, the user can read the bearing directly through the prism.

**Use of a prismatic compass:**

A prismatic compass is used to determine the direction or bearing of a particular point or object. It is commonly used in surveying to measure angles and directions between points. It is also used in navigation to determine the direction of travel.

**Agate cap and its role:**

The agate cap is a protective cover or cap that is fitted on top of the prismatic compass. It is typically made of agate, a semi-precious stone known for its hardness and durability. The agate cap helps to protect the compass from damage and ensure its accuracy. It also provides a stable base for the compass, allowing it to be easily held and used by the surveyor or navigator.

In conclusion, an agate cap is fitted with a prismatic compass. The prismatic compass is a type of compass commonly used in surveying and navigation. It consists of a magnetic needle, a graduated circle, and a prism. The agate cap serves to protect the compass and provide a stable base for its use.