The correct answer is C. both (A) and (B).
According to application, instruments are classified as switch board instruments and portable instruments.
Switch board instruments are designed to be installed in switch boards and other fixed locations. They are usually large and heavy, and they are designed to be operated by trained personnel.
Portable instruments are designed to be carried and used by personnel in the field. They are usually smaller and lighter than switch board instruments, and they are designed to be operated by personnel with a basic level of training.
Option A is correct because switch board instruments are used in switch boards.
Option B is correct because portable instruments are used in the field.
Option C is the correct answer because it includes both Option A and Option B.
Option D is incorrect because moving coil instruments are a type of instrument, not a type of application.
Option E is incorrect because moving iron instruments are a type of instrument, not a type of application.