**Phasor Diagram for Open Circuit Characteristic of Alternator**

The correct answer is option 'A', which states that the phasor diagram best describes the open circuit characteristic of an alternator. Let's understand why this is the correct answer.

**Explanation:**

An alternator is a device that converts mechanical energy into electrical energy. It consists of a rotor (rotating part) and a stator (stationary part). When the rotor is rotated, it induces electromotive force (EMF) in the stator windings, which generates electrical power.

The open circuit characteristic of an alternator refers to the relationship between the terminal voltage of the alternator and its field current when there is no load connected to the alternator. In other words, it represents the behavior of the alternator when there is no external electrical load connected.

**Phasor Diagram:**

A phasor diagram is a graphical representation of the amplitude and phase relationship between different quantities in an AC circuit. It is a vector diagram that helps visualize the relationship between voltage and current in an AC circuit.

In the case of the open circuit characteristic of an alternator, the phasor diagram represents the relationship between the terminal voltage (Vt) and the field current (If) of the alternator.

**Interpretation of Phasor Diagram:**

The phasor diagram for the open circuit characteristic of an alternator will have the following features:

1. The phasor representing the terminal voltage (Vt) will be the reference phasor and will be oriented along the horizontal axis (real axis).

2. The phasor representing the field current (If) will be oriented at an angle with respect to the reference phasor. The angle represents the phase difference between the terminal voltage and the field current.

3. The length of the field current phasor represents the magnitude of the field current.

4. The phasor diagram may also include the phasor representing the armature reaction, which is the magnetic field produced by the armature windings of the alternator. This phasor will be oriented at a different angle with respect to the reference phasor.

**Conclusion:**

Based on the above explanation, it is clear that the phasor diagram best describes the open circuit characteristic of an alternator. The phasor diagram represents the relationship between the terminal voltage and the field current of the alternator in an open circuit condition.