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The correct answer is "half silvered and half unsilvered."

In an optical square, the horizon glass is half silvered and half unsilvered. This design allows the user to see both the reflected image and the direct view through the glass, aiding in precise alignment and measurement.

The correct answer is "hill."

A hill is an obstacle that obstructs vision but does not prevent chaining (measuring distances). Rivers and ponds obstruct both vision and chaining.

The correct answer is "converge to the poles."

True or geographical meridians through various stations converge toward the poles. This means that all meridians intersect at both the North and South Poles, making them non-parallel and converging as they approach the poles.

The correct answer is "magnetic decline."

The horizontal angle between the true meridian and the magnetic meridian is known as magnetic decline, also referred to as magnetic declination. This angle indicates the deviation of the magnetic north from the true north.

The correct answer is "right hand upright."

When the object lies on the left-hand side of the chain line, the optical square is held in the right hand upright to correctly take the offset.

The correct answer is "none of the above."

In the whole circle bearing (WCB) system, the bearing of a line is measured clockwise from the north point of the reference meridian towards the line right round the circle. The bearing is expressed in degrees, ranging from 0° to 360°.

The correct answer is "dip."

The inclination of the magnetic needle of the prismatic compass downward towards the pole due to the Earth's magnetic influence is known as the dip or magnetic dip. It is the angle that the magnetic needle makes with the horizontal plane.

The correct answer is "all of the above."

All the statements listed are incorrect:

1. The magnetic meridian coincides with the true meridian at all the places - This is false because the magnetic meridian (direction of the magnetic needle) does not generally coincide with the true meridian (geographical north-south line) except at certain points.

2. The magnetic meridian does not vary from place to place on the earth's surface - This is false because the magnetic meridian does vary depending on the location due to variations in the Earth's magnetic field.

3. The magnetic declination at a place is constant - This is false because magnetic declination (the angle between magnetic north and true north) changes over time and varies with location.

The correct answer is "the surveys extend over small areas."

In plane surveying, the curvature of the Earth is generally ignored because the surveys extend over small areas where the curvature has a negligible effect. This assumption simplifies calculations and is suitable for most engineering and construction projects conducted over relatively short distances.

The correct answer is "80 cm."

An average length of a pace is typically around 80 cm (0.8 meters). This can vary slightly depending on the individual's stride length, but 80 cm is a common average used in surveying and measurement.