What must be true for natural selection to happen? apex?

For a natural selection to occur, there must be genetic variation by mutations. Application of environmental pressures for adaptation and over-production of the offsprings also allows the occurrence of natural selection. 
Natural selection allows only those organisms to survive, who adapt or change themselves, in accordance with the change in all environmental and physiological conditions. 

You e got the answer to the question right

Idon't know how to do this but maybe ask your mom or dad