How many strain gauges are required for measuring deformation? 

The correct option is 1

Concept:

A strain gauge is a device used to measure the strain (deformation) of an object. The most common type of strain gauge, which consists of an insulating flexible backing supporting a metallic foil pattern, works on the principle that the electrical resistance of the wire changes in response to changes in its length.

• The number of strain gauges used depends on what type and complexity of the measurement you are looking to achieve.
• One strain gauge can be used to measure strain along a single axis. This is the simplest setup and is used in "uniaxial" strain measurements.
• Two strain gauges can be used when you want to cancel out the effects of temperature. The second strain gauge is set up in a way that it only experiences the temperature change, but no strain.
• Three strain gauges can be used for more complex measurements such as finding out the principal strain in a plane.

However, four strain gauges are typically used to form a Wheatstone bridge. A Wheatstone bridge is a configuration that is used extensively for measuring resistance and therefore strain changes. Using four strain gauges (in what's called a "full bridge" configuration) provides temperature compensation, increased sensitivity, and the ability to detect strain in more than one direction.

Types of strain gauges: