Step 1: Understand ribosomes. Ribosomes are tiny cell parts (organelles) found in both simple and complex cells. They have no membrane around them.
Step 2: Recall their nickname. Ribosomes are often called the protein factories of the cell because their main job is to make proteins.
Step 3: See how they work. A ribosome reads the message carried by mRNA and joins amino acids one by one in the right order to build a protein. This process is called translation.
Step 4: Note where they are found. Some ribosomes float freely in the cytoplasm, while others stick to the rough endoplasmic reticulum. Both kinds make proteins.
Step 5: Rule out the other options. Lipids are mostly made in the smooth endoplasmic reticulum, carbohydrates form through other pathways, and nucleic acids are made in the nucleus.
Step 6: Conclude. So ribosomes are the sites where proteins are made. \[ \boxed{\text{Proteins}} \]