In which part of the cell does most of the metabolic reactions take place?

The correct answer is cytoplasm. 

Most of the metabolic reactions in the cell, including cellular respiration, protein synthesis, and the breakdown of molecules for energy, occur in the cytoplasm. The cytoplasm contains enzymes and other molecules that are necessary for these reactions to occur. 

The nucleus contains DNA and is involved in transcription and replication, while mitochondria are involved in cellular respiration and energy production. The cell membrane is involved in transport and communication but not in metabolic reactions.