Speciation has definitely occurred when two groups of related organisms
a. cannot interbreed successfully
b. have drastically different behaviors
c. have drastically different behaviors
d. become permanently isolated from each other

The right option is; A. Cannot interbreed successfully

Speciation is a process within evolution in which new distinct species that are reproductively isolated from one another are formed. Speciation involves the disintegration of a single parent species into two or more genetically distinct species. During the process of speciation, species may not be able to interbreed successfully due to behavioral, genetic, or physical differences that arise between the new species.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.