To construct a line segment, we need instruments that can accurately measure and draw straight lines. Among the given options, the correct answer is option A: compasses and a scale.

1. Compasses:
- Compasses are a drawing instrument used to create circles or arcs of a specific radius.
- In the context of constructing a line segment, compasses are used to create the desired length of the line segment.
- By setting the compasses to the desired length on a scale, we can mark off the endpoints of the line segment.

2. Scale:
- A scale is a measuring instrument with evenly spaced marks or divisions used to measure or draw straight lines.
- To construct a line segment, we need to measure the desired length accurately, and a scale provides a precise measurement.
- By aligning the scale with the endpoints marked using the compasses, we can draw a straight line segment connecting them.

The other options provided (b, c, and d) do not include the necessary instruments for constructing a line segment.

- Option B: A divider and a protractor:
- A divider is used to measure distances, but it cannot directly draw straight lines.
- A protractor is used to measure angles, not lengths.

- Option C: Set squares and scale:
- Set squares are right-angled triangles used for drawing perpendicular or parallel lines, but they do not measure lengths.
- A scale, as mentioned earlier, is necessary for accurate measurements.

- Option D: A divider and compasses:
- While compasses are useful for constructing a line segment, a divider alone cannot draw straight lines.

In summary, to construct a line segment, we need instruments that can accurately measure and draw straight lines. Among the given options, compasses and a scale are the appropriate instruments for this purpose.

Option A is true because a Set Square is not a device to construct a line