The unit of electric charge is ampere-second (A·s), which is derived from the definition of current. Electric charge is the product of current and time, where: \[ \text{Electric Charge (Q)} = \text{Current (I)} \times \text{Time (t)}. \] Therefore, the unit of electric charge is ampere-second (A·s). This makes option (C) the correct answer. The other options correspond to:
Electric current, which has the unit ampere (A).
Electric potential, which has the unit volt (V).
Electromotive force, which also has the unit volt (V).
The correct option is (C): electric charge
Figure 1 shows the configuration of main scale and Vernier scale before measurement. Fig. 2 shows the configuration corresponding to the measurement of diameter $ D $ of a tube. The measured value of $ D $ is: