Based on figure 21-1 ,animals are most closely related to

The correct answer is-

Choanozoa.

A phylogenetic tree is a representation of evolutionary relationship between different species. It is constructed on the basis of their similarities and differences in their morphological ( physical traits) as well as genetic characteristics.

Two species are said to be most closely related if they share most recent common ancestor. In other words, when two species are descended from a most recent common ancestor, they are said to be most closely related with each other.

In this question, Animals and Choanozoa share most recent common ancestor.

Thus, animals are most closely related to Choanozoa.

a covalent bond, specfically triple bond