Final answer:
The test charge q in the definition of the electric field must be vanishingly small in order to simplify the calculation of the electric field.
Step-by-step explanation:
The test charge q in the definition of the electric field must be vanishingly small in order to simplify the calculation of the electric field. By assuming that the test charge is vanishingly small, we can ignore any interactions between the test charge and the charge creating the electric field.
This allows us to focus solely on the charge creating the field, represented by Q, and ensures that the electric field is only dependent on the charge creating it, rather than the test charge.