< b="" />Introduction: < />
A plug gauge is a precision measuring instrument used to measure the size of a hole. It is commonly used in industries such as manufacturing, engineering, and quality control. Plug gauges are designed to provide accurate and reliable measurements of the diameter or size of a hole, ensuring that it meets the required specifications.

< b="" />Explanation: < />
The correct answer to the question is option 'B', which states that a plug gauge is used to measure hole size. Here's why:

< b="" />Shaft size: < />
A plug gauge is not used to measure the size of a shaft. For measuring shaft size, various other tools such as micrometers, calipers, or snap gauges are commonly used. These tools are specifically designed to measure the outer dimensions or diameters of objects like shafts.

< b="" />Wire thickness: < />
A plug gauge is not used to measure wire thickness. For measuring wire thickness, tools like wire gauges or micrometers are commonly used. These tools are specifically designed to measure the diameter or thickness of wires accurately.

< b="" />Depth of threads: < />
A plug gauge is not used to measure the depth of threads. To measure the depth of threads, thread depth gauges or thread micrometers are commonly used. These tools are designed to measure the depth or pitch of threads accurately.

< b="" />Hole size: < />
A plug gauge is primarily used to measure the size of a hole. It consists of a cylindrical body with precisely calibrated dimensions. The gauge is inserted into the hole, and if it fits perfectly without any play or excessive tightness, it indicates that the hole is within the required specifications. The gauge is usually marked with its corresponding size, allowing the user to determine the diameter or size of the hole.

< b="" />Conclusion: < />
In conclusion, a plug gauge is specifically designed to measure the size of a hole. It is not used for measuring shaft size, wire thickness, or depth of threads. By using a plug gauge, manufacturers and engineers can ensure that the holes they produce meet the required specifications, ensuring the quality and functionality of the final product.