Question

what is a reproductive barrier? an obstacle preventing two species from producing viable, fertile offspring a physical disorder in an individual that helps them reproduce more efficiently a large portion of older individuals in a population that reproduce a lot

Explanation:

A reproductive barrier is defined as something that stops two species from producing viable and fertile offspring. The second option is incorrect as a reproductive barrier does not help in more efficient reproduction. The third option is about a population's age - reproduction aspect, not a barrier.

Answer:

an obstacle preventing two species from producing viable, fertile offspring