We can use a thread to measure the length of a curved line.

And Thread is not a option in this question.

So, Option 'D' none of these

Explanation:

A curved line is a line that is not straight, it has a curve or bend to it. When measuring the length of a curved line, certain objects may not be suitable for accurate measurements. Let's analyze each option to determine which one cannot be used for measuring the length of a curved line.

a) Only ruler:
A ruler is a common measuring tool that is used to measure straight lines or objects. It consists of a straight edge with marked units of length. While a ruler can be used to measure the length of a curved line, it may not provide accurate measurements due to the curve or bend in the line. Therefore, a ruler can be used, but it may not be the most precise tool for measuring a curved line.

b) Screw gauge:
A screw gauge, also known as a micrometer, is a precision measuring instrument commonly used in engineering and scientific applications. It is used to measure the thickness or diameter of small objects with high accuracy. However, a screw gauge is not designed to measure the length of curved lines. Its mechanism is more suitable for measuring small distances or dimensions.

c) Set square:
A set square is a triangular ruler that is used in geometry and technical drawing. It has two legs with marked angles and a hypotenuse. Set squares are primarily used for drawing straight lines and right angles. While they can be used to measure the length of straight lines, they are not suitable for measuring curved lines due to their rigid shape.

d) None of them:
The correct answer is option 'D' - None of them. None of the objects mentioned (ruler, screw gauge, set square) are specifically designed for measuring the length of curved lines. While a ruler can be used to approximate the length, it may not be accurate. Other specialized tools such as a flexible measuring tape or a string can be more suitable for measuring curved lines.

In conclusion, when measuring the length of a curved line, it is best to use specialized tools such as a flexible measuring tape or a string rather than a ruler, screw gauge, or set square. These specialized tools can conform to the curve of the line and provide more accurate measurements.