The correct answer is D. Possession of facial hair in adult humans.

Sex-linked traits are characteristics influenced by genes located on the sex chromosomes (X and Y). Facial hair in humans is a sex-linked trait because:

- The gene controlling facial hair growth is located on the X chromosome.

- Males, with their XY chromosomes, are more likely to inherit the dominant allele for facial hair growth.

- Females, with their XX chromosomes, would need to inherit two copies of the dominant allele (one from each X chromosome) to express facial hair growth.

The other options are not sex-linked traits:

A. Ability to grow long hair in females is influenced by multiple genes, but it's not sex-linked.

B. Colour of the skin in humans is influenced by multiple genes and is not sex-linked.

C. Ability to roll the tongue is an autosomal dominant trait, meaning it's not linked to the sex chromosomes.