Which successional stage lasts longest? (ecological succession)

The correct answer is primary succession.

Explanation:

Ecological succession has three major parts; primary succession, secondary succession, and the climax site. The longest part or the stage of ecological succession is primary succession.

Primary succession is one of the ecological succession series of community changes that takes place on a completely new habitat which has never been acquired or colonized before this.  

Thus, The correct answer is primary succession.

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.