Explanation:

To understand why the correct answer is option 'A', let's first understand what is meant by the 'fix' of a plane table.

Plane Table:
A plane table is a surveying instrument used for mapping and measuring horizontal distances and angles. It consists of a flat table with a mounted alidade (a sighting device) and a ruler. The table is leveled and oriented to the magnetic north using a compass.

Fix of a Plane Table:
The fix of a plane table refers to the process of determining the accurate position of the table at a particular point on the ground. This is done by sighting and measuring angles to at least three known points on the ground from the table.

Conditions for a Good Fix:
In order to obtain an accurate fix of the plane table, certain conditions need to be met. One such condition is the position of the middle station in relation to the other two known points.

The correct answer is option 'A' - The middle station is nearest. This means that the middle station should be the closest to the plane table among the three known points.

Here's why this condition leads to a good fix:

1. Parallax Effect:
When sighting a known point from the plane table, there can be a parallax effect if the point is too far away. Parallax is the apparent shift in the position of an object when viewed from different angles. This can introduce errors in the measurement of angles, leading to an inaccurate fix. Therefore, it is desirable to have the known points relatively closer to the plane table.

2. Accuracy of Measurements:
When the middle station is nearest, the angles measured to the known points will be larger compared to the case when the middle station is farthest. Larger angles result in more accurate measurements as the smallest error in measuring an angle is amplified when the angle is small.

3. Stability of the Plane Table:
Having the middle station nearest to the plane table ensures better stability of the instrument. As the plane table is mounted on a tripod, having the middle station closer reduces the chances of any movement or shaking of the table due to external factors such as wind or uneven ground.

Therefore, having the middle station nearest to the plane table ensures a good fix by minimizing parallax effect, improving measurement accuracy, and enhancing the stability of the instrument.