Gibbons live in monogamous pair bonds and are therefore

This question is unfortunately incomplete as the following options are provided:
a.  not dimorphic with respect to body size
b.  a solitary species
c.  have intense male competition for mates
d.  better adapted to life in the forest

The answer is a: not dimorphic with respect to body size. Monogamy is thought to lower pre- and post- copulatory competition between males. This means that there is less selection for morphological characteristics in males that would provide a sexually selective advantage for mating, such as size

This type of galaxy is an approximately ellipsoidal shape and a smooth, nearly featureless image. they are the largest single star systems, in the universe and are built by the collisions of many smaller galaxies.