**Core Concept**
ATP synthetase is an enzyme complex that plays a crucial role in generating energy for the cell by catalyzing the synthesis of ATP from ADP and inorganic phosphate. The enzyme is embedded in the inner mitochondrial membrane, where it harnesses the energy from the proton gradient to produce ATP.

**Why the Correct Answer is Right**
The correct answer is B. Mitochondria because ATP synthetase is an essential component of the mitochondrial electron transport chain. The enzyme is embedded in the inner mitochondrial membrane, where it utilizes the proton gradient generated by the electron transport chain to drive the synthesis of ATP. This process is critical for cellular respiration and energy production in the cell. The enzyme is specifically found in the mitochondria and is not present in other cellular organelles such as the Golgi apparatus, cytosol, or endoplasmic reticulum.

**Why Each Wrong Option is Incorrect**
**Option A:** The Golgi apparatus is involved in protein modification, sorting, and packaging, but it does not contain ATP synthetase. The Golgi apparatus is distinct from the mitochondria and is not involved in energy production.
**Option C:** The cytosol is the region between the cell membrane and the nuclear envelope, but it does not contain ATP synthetase. The cytosol is involved in various cellular processes such as protein synthesis, metabolism, and signaling, but it is not responsible for energy production.
**Option D:** The endoplasmic reticulum is a network of membranous tubules and cisternae that is involved in protein synthesis, folding, and transport, but it does not contain ATP synthetase. The endoplasmic reticulum is distinct from the mitochondria and is not involved in energy production.

**Clinical Pearl / High-Yield Fact**
The mitochondria are often referred to as the "powerhouses" of the cell because they generate most of the energy that the cell needs to function. ATP synthetase is a critical component of the mitochondrial electron transport chain and plays a key role in energy production in the cell.

**✓ Correct Answer: B. Mitochondria**