Final answer:
A deletion is a type of chromosome mutation that results in a loss of genetic material from a chromosome, potentially leading to serious effects on an organism. The correct option is a).
Step-by-step explanation:
The type of chromosome mutation that can decrease the amount of genetic material for one entire chromosome is deletion. A deletion occurs when a segment of a chromosome is lost, which means the genes on that segment are also lost.
This can happen during the process of cell division if a piece of the chromosome does not properly reattach or does not rejoin at all.
Chromosome deletions can have significant effects, including a genetic imbalance that may be lethal to a diploid organism depending on the size and location of the deletion. Option a) is the correct one.