Even after attaining sexual maturity, larval characters are retained. It is known as

The correct option is Neoteny.

Explanation:

In developmental biology, the retention of larval or juvenile characteristics in an adult, sexually mature organism is known as neoteny (also referred to as juvenile behavior or juvenile features retention). A classic example of this is seen in the Axolotl salamander, which retains its larval external gills and aquatic lifestyle even after reaching sexual maturity.
Let's define the other terms to understand why they are incorrect:

1. Phylogenesis refers to the evolutionary history and development of a species or taxonomic group of organisms, rather than the development of an individual organism.
2. Parthenogenesis is a form of asexual reproduction where offspring develop from unfertilized eggs.
3. Ontogenesis (or ontogeny) is the entire developmental history of an individual organism from the fertilization of the egg to its mature adult form.

Thus, the retention of larval characters after attaining sexual maturity is specifically called Neoteny.