Which of the following is the most precise instrument for measuring length?

Options

A. Metre rod of least count 0.1 cm
B. vernier callipers of least count 0.01 cm
C. Screw gauge of least count 0.001 cm
D. none of these

Explanation

To determine which instrument is the most precise for measuring length, we need to analyze the least count of each instrument provided in the options. The least count is the smallest measurement that can be accurately read with an instrument; the smaller the least count, the more precise the instrument.

### Step-by-Step Solution:

1. **Identify the Instruments and Their Least Counts:**
- Meter rod: Least count = 0.1 cm
- Vernier caliper: Least count = 0.01 cm
- Square gauge: Least count = 0.001 cm

2. **Compare the Least Counts:**
- The least count of the meter rod is 0.1 cm.
- The least count of the vernier caliper is 0.01 cm.
- The least count of the square gauge is 0.001 cm.

3. **Determine Which Least Count is the Smallest:**
- Among the three least counts, 0.001 cm (square gauge) is the smallest.
- 0.01 cm (vernier caliper) is larger than 0.001 cm.
- 0.1 cm (meter rod) is the largest.

4. **Conclusion:**
- Since the square gauge has the smallest least count, it is the most precise instrument for measuring length.

### Final Answer:
The most precise instrument for measuring length is the **square gauge**.

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