Duplication is a chromosomal mutation in which part of the chromosome breaks off and is incorporated into its a) homologous chromosome.
b) parent chromosome.
c) nonhomologous chromosome.
d) base pair.

homologous chromosome

Explanation:

an extra copy is made after a duplication mutation

hope this helps

Each new cell will have four chromosomes

Explanation:

In mitotic cell division, one cell divides to produce two identical cells with exactly the same amount of genetic amount i.e chromosome as that of the parent cell.  

In this case the total numbers of chromosomes in parent cells are four which means each new cell will have four chromosome i.e exact amount of genetic material.