6. before transferring sperm to the female during mating, the males of some species of beetles use their copulatory organs to remove the sperm of other males. in cases in which a female mated first with a male of a different species, the male of her own species is particularly adept at removing the other male’s sperm. this is an example of isolation.
a. geographical
b. temporal
c. mechanical
d. postzygotic
e. precoital

Mechanical Isolation

Explanation:

Mechanical isolation is a type of barrier that prevents the species that are closely related from mating with each other.

It is used in case of plants and animals(here beetles) in which the without compatible sex organs individual of different species will not be able to mate and produce offspring.

Thus, the correct answer is option C

reduction in birds skeleton

Explanation:

The reduction in a bird's skeletal weight is largely a result of hollow construction of the skeleton. The fusion or loss of individual bones reduces the number of movable joints in the bird's skeleton, making it rigid and better adapted as the internal framework of a flying machine.