Which mother is at greatest risk of having a child with a chromosomal abnormality?

I believe the answer is: a 40-year-old mother
Chromosomal abnormalities refers to a condition when the mother failed to assign proper DNA sequence to the infant.
For average women, the chance of getting chromosomal abnormalities would increase significantly after she passed the age of 35.

Alloy of metals is the answer

The answer should be b