Alidade is an important instrument used in surveying to measure horizontal angles. It consists of a telescope mounted on a tripod, with a circular scale attached to it. The alidade can be rotated horizontally to measure angles accurately.

The correct answer is option 'A' - Plane table survey. In a plane table survey, the alidade is used to measure the horizontal angles between various points on the ground. This surveying technique involves setting up a plane table at a known point and then sighting various targets using the alidade. By measuring the angles between these targets, the surveyor can create a detailed map or plan of the surveyed area.

The alidade is used in conjunction with other surveying tools such as a tape measure and a plumb bob to accurately determine the positions of various points on the ground. It is particularly useful in situations where the terrain is relatively flat and the targets are visible from the plane table.

Here is a breakdown of the different surveying techniques mentioned in the options:

a) Plane table survey: This surveying technique employs the use of an alidade to measure horizontal angles and create a map or plan of the surveyed area.

b) Contour survey: This surveying technique is used to determine the elevation and shape of the land by measuring and mapping contour lines. While an alidade may not be explicitly used in a contour survey, it can still be employed to measure horizontal angles in certain situations.

c) Reconnaissance survey: This type of survey is a preliminary survey conducted to gather information about an area before a detailed survey is carried out. It typically involves visual inspection and basic measurements, and the use of an alidade may not be necessary.

d) Archaeological survey: This type of survey is conducted specifically for archaeological purposes, such as locating and documenting archaeological sites. While an alidade may be useful in certain aspects of an archaeological survey, it may not be the primary instrument employed.

In conclusion, the survey that employs an alidade is a Plane table survey. The alidade is used to measure horizontal angles and create a detailed map or plan of the surveyed area.