Mutations that occur without a known cause are known as:
a. spontaneous mutations
b. induced mutations
c. harmful mutations
d. adaptive mutations
e. reversion mutations

a. spontaneous mutations

Explanation:

Spontaneous mutations occur randomly in all cells and without any agents. Induced mutations are the result of some exposure to a physical or chemical agent called a mutagen. Mutations can be classified as harmful and adaptive regardless of whether they were spontaneous or induced mutations. The reversal mutation is when a given mutation is reversed by another mutation so that the nucleotide is changed back to its original state.