A standard unit should be
(a) consistent
(b) reproducible
(c) invariable
(d) easily available for usage

Options

A. Only a & b are true
B. Only a & c are true
C. only a, c & d are true
D. a, b , c d are true

Answer

To determine the characteristics of a standard unit, we need to analyze each option provided in the question:

1. **Consistent**: A standard unit must be consistent, meaning that it should represent the same quantity under all conditions. For example, one meter should always equal one meter, regardless of where or how it is measured.

2. **Reproducible**: A standard unit should be reproducible. This means that if the same measurement is taken multiple times, it should yield the same result. For instance, if we measure a length of 2 meters, we should be able to reproduce that measurement reliably.

3. **Invariable**: A standard unit must be invariable, meaning it should not change over time or under different conditions. For example, the definition of a meter should remain the same regardless of environmental factors.

4. **Easily Available for Usage**: A standard unit should be easily available for use. This means that it should be accessible and practical for people to use in various situations.

Now, let's summarize the analysis:

- All four characteristics (consistent, reproducible, invariable, and easily available) are essential for a standard unit.

Thus, the correct answer is that all options (a, b, c, and d) are true.

### Final Answer:
**(d) a, b, c, d all are true.**

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