The organelle that converts nutrients into energy in an animal cell is the mitochondrion.

Here's why:

* Mitochondria are the powerhouses of the cell: They are responsible for cellular respiration, a process that breaks down glucose (a type of sugar) and other nutrients in the presence of oxygen to produce ATP. ATP is the primary energy currency of cells.

* Other organelles play different roles: While other organelles like the nucleus, ribosomes, and Golgi apparatus are essential for cell function, they don't directly convert nutrients into energy.