Which of the following would not be considered a stage in speciation?

A population acquired new traits making it different from a previously similar to population

Don't know what your choices are, but acquiring new traits that eventually make the population different from a previously similar population IS a stage of speciation. The population acquire traits when they are learning to adapt to the new environment after they have been separated from the other part of the population that used to be similar to them. Eventually, the genetics become so different due to the different traits being reproduced between the two populations that they become two new species and can no longer produce fertile offspring. They can not interbreed.

If interbreeding or fertile offspring being produced are part of your choices, that choice would represent the "not be considered a stage in speciation".