How does competition increase stability in an ecosystem?
a a decreases biodiversity
b it keeps population balanced
c it causes cyclical disturbances
d it removes keystone species

The competition increase stability in an ecosystem by keeping population Balanced.  

Explanation:  

There is increase in competition this will in turn deals with stability in ecosystem. Because this creates continuous food chain where a larger animal will eat the smaller one and this cyclical chain keeps in check. The food chains creates complexities in the biodiversity this again is compensated with increase in population. With a lot more increase in species again the food chain demands the cyclical procedure. This is how in our biodiversity the stability in the organism is maintained.  

it's c. (to reduce the greenhouse effect/carbon gasses)