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