Those mutations that arise in the absence of known mutagens are known as:

• Spontaneous mutations are changes in the DNA sequence that occur naturally, without the involvement of any identifiable external mutagen. These mutations can arise due to various internal processes, such as:
● Errors during DNA replication, like DNA polymerase mistakes or misincorporations.
● Chemical reactions within the cell that damage DNA, such as spontaneous deamination or oxidation.
● Errors during DNA repair mechanisms that lead to incorrect repair of damaged DNA.

• Explanation of 📌Other Options:
● a) Induced mutations: These are mutations caused by exposure to known mutagens, such as radiation, chemicals, or viruses.
● b) Fused mutation: This is not a recognized term in genetics. It might be a misspelling of "fusion mutation," which refers to a specific type of chromosomal rearrangement where two chromosomes join abnormally. However, it doesn't relate to the absence of mutagens.