First question:
a. Gastrulation
Gastrulation is a phase of the embryonic development in animals. It doesn't occur in plants or any other organism. The other options can be put apart because multicellularity, sexual reproduction, and flagellated sperm are also characteristics that are present in plants, for example. Heterotrophic nutrition is something that's part of all animals as well as non green plants.
Second question:
a. Multicellularity
Synapomorphy is a shared trait that distinguishes a clade from other organisms. Choanoflagellates are considered the most close ancestors to the animals distinguishing from them by multicellularity. They are very studied with the purpose to know the mechanisms of colonies' formation and evolution to multicellular animals.
Third question:
d. Choanoflagellates
This are the most closely related living protist group. The choanoflagellates are very similar to the choanocytes of the sponges bringing them into closeness with the animals. They even aggregate in colonies, and are thought, because of all of this, to be the closest branch of single cells to that of the animals.
