When a coil is switched off, a large BACK EMF appears across the coil.
This back emf may be several thousand volts in value, enough to destroy the transistor.
The diode, which is normally reverse biased, is forward biased by the back emf,and conducts, its low resistance short circuiting the back emf and protecting the transistor.

Diagram B
This circuit consists a meter movement with two germanium diodes across it.

Typical movements require only 100 mV for full scale deflection of the pointer.
If a voltage, exceeding about 0.3 volts, appears across the circuit then one of the diodes will conduct and protect the movement from excessive voltage.