Mutation occurs at the time of:

The correct answer is:

Cell division.

• When the normal sequence of the genetic material is altered, the phenomenon is called mutation
• Cell division is the process of producing daughter cells
• During cell division, the contents of the cytoplasm and nucleus multiply
• Inside the nucleus, the DNA will undergo replication
• During replication, errors that occur will be removed by "proof reading mechanism" present in cells
• Failure of this mechanism or due to environmental factors the normal sequence of nucleic acid changes 

• Mutation can be due to genetic or environmental factors
• The cause of mutation is called mutagen
• The individual affected is called mutant
• Not all mutations are harmful