D

Step 1: Define the processes of electron emission in Column I:
(i) Thermionic emission occurs when electrons are emitted from a material due to thermal energy, meaning the material is heated to a high temperature.
(ii) Photoelectric emission happens when light photons strike a material and provide enough energy to dislodge electrons from it.
(iii) Field emission refers to the emission of electrons from a conductor when subjected to a very strong electric field.
(iv) Secondary emission occurs when incoming charged particles (like electrons) collide with a material and cause the emission of additional electrons.

Step 2: Match these types of emissions with their methods in Column II:
- (i) Thermionic emission: By heating (D)
- (ii) Photoelectric emission: By irradiating with light (A)
- (iii) Field emission: By applied strong electric field (B)
- (iv) Secondary emission: By colliding accelerated electrons on metals (C)

Final Match:
(i) - D, (ii) - A, (iii) - B, (iv) - C.

Therefore, the final answer is: D.