Question

builds proteins, each cell contains thousands, found on er and floating throughout cell

Explanation:

Ribosomes are the cellular structures responsible for protein synthesis. They are numerous in cells and can be found attached to the endoplasmic reticulum (ER) or free - floating in the cytoplasm. The nucleus is the control center of the cell containing genetic material. A vacuole is mainly involved in storage (e.g., water, nutrients, waste). The Golgi body is involved in modifying, sorting, and packaging proteins and lipids.

Answer:

Ribosomes