Competitive exclusion occurs when a.. organisms attempt to fill the same niche. b.. organisms mark their territory. c.. organisms experience intraspecific aggression. d.. organisms have a symbiotic relationship

Ans. (a).organisms attempt to fill the same niche.

Competitive exclusion can be defined as elimination of one or more different species from a habitat that share identical need for resources, such as food and shelter.  

It occurs due to competition between organisms of two species present in same habitat or niche for the same limiting resource (s). It results in either extinction of weaker competitive species or he will shift to a new habitat.

Thus, the correct answer is option (a).

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands