Concept:The sex of a mammal is fixed by the type of sperm that fertilises the egg.
Explanation:A mammal receives one sex chromosome from each parent at fertilisation.
An egg always carries an X chromosome.
A sperm may carry either an X or a Y chromosome.
If a Y-bearing sperm fertilises the egg, the offspring is XY and becomes male.
If an X-bearing sperm fertilises the egg, the offspring is XX and becomes female.
This genetic combination is completed at the moment of conception.
Therefore, no subsequent stage such as birth, puberty, or adolescence can change or set the sex.
Answer:D. at conception