Chromosomal deletion is the loss of genetic code, caused by a segment of
chromosome breaking away during DNA replication. The best answer is the
third one, "when part of a chromosome breaks off and does not
reattach."
The first answer, "part of a chromosome breaks off
and is inserted into the middle of another", describes chromosomal
insertion.
The second answer, "part of a chromosome breaks off and reattaches backward", describes chromosomal mutation.
The final answer, "part of a chromosome breaks off and attaches to another", describes chromosomal translocation.
