The correct answer is option b.They can mate and produce offspring that can mate.
As per biological species concept which is the most accepted theory in regards to species concept, organisms which can interbreed to produce fertile off-springs constitute a species. So it means that emperor penguins constitute a species because they can mate and produce off-springs that can mate further to enhance their generations. Merely ability to reproduce is not enough to categorize animals into a particular species, their off springs must also be able to reproduce and enhance their generations.
For example, a tigon is a hybrid which is born when a male tiger and a female lion mate with each other. But lions and tigers are still considered as different species because although their mating results into an off-spring yet that off-spring i.e. tigon is not fertile. A tigon is unable to mate further.
