The correct answer is option 3, cytoplasm. In a cell, glucose is converted into pyruvate through a process known as glycolysis. Glycolysis occurs in the cytoplasm, which is the fluid-filled area between the cell membrane and the nucleus.

Option 1, mitochondria, is incorrect because the conversion of glucose into pyruvate occurs before entering the mitochondria. Pyruvate is later transported into the mitochondria for further energy production.

Option 2, nucleus, is also incorrect because the nucleus is responsible for storing genetic material and directing cellular activities, but not directly involved in glucose metabolism.

Option 4, endoplasmic reticulum, is also incorrect as it is primarily involved in protein synthesis and lipid metabolism, not the conversion of glucose into pyruvate.

Therefore, the correct answer is option 3, cytoplasm, where glycolysis takes place.