Discontinuous variations are
(a)Essential features
(b)Acquired characters
(c)Non-essential changes
(d)Mutations

Hint: The slight variations that happen among cells, organisms, or to a group or species which are either due to genetic modification or by environmental influences are referred to as variations.

Complete answer:
Discontinuous variations are formed due to alterations in the genome or genes, which occur occasionally and disturb the genetic construction of the organisms. These variations are also called mutations.
-Mutations can be caused by external factors or agents known as the mutagens. Mutagens are characterized into three types: chemical mutagens, physical mutagens, and biological mutagens.

Additional Information: Chemical mutagens cause a mutation by mimicking the genetic material components or by substituting them. A common example of chemical mutagens are benzene, sodium azide, aromatic amines, alkaloids, and deaminating agents
Physical mutagens are any physical component which can alter the genetic content by breaking them or by altering the bonding of the genetic material. Major examples of physical mutagens are X rays, gamma rays, and UV rays.
Biological mutagens are agents that can change the genetic content in the desired way, for example, bacteriophages that alter the genetic component of the infected host to the needs of the virus propagation.
So, the correct answer is ‘Mutations’.

Note: -Mutations are of different types, in certain mutations the organism undergoes changes in genetic content but it does not affect the transcription and translation process, as one protein can be translated using more than one codon.
-These kinds of mutations are known as a silent mutation, where there won't be any morphological changes or changes in protein synthesis even though changes occurred in the genetic component.
-Not all mutagens are carcinogens, but all carcinogens are mutagens.