Final answer:
The mutation described is a deletion, where a portion of a chromosome is lost due to breakage and incorrect rejoining.
Step-by-step explanation:
When a chromosome is exposed to a mutagen and breaks into two pieces, followed by the removal of a portion from one of the ends before the two pieces rejoin, the mutation that occurs is known as a deletion. This type of chromosomal mutation involves the loss of genetic material, which can have various effects on an organism depending on which genes are removed.
Deletions, along with other types of chromosomal alterations such as duplications, inversions, insertions, and translocations, can lead to significant genetic variation or cause genetic disorders.