Question

controls and regulates the activities of the cells. disassembles and reforms each time when cells divide. spindle fibers nucleus nuclear membrane chromosomes

Explanation:

The nucleus is the control center of the cell, regulating cellular activities. During cell division (mitosis or meiosis), the nuclear membrane disassembles (at the start of mitosis, for example, in prophase) and reforms (in telophase). Spindle fibers are involved in moving chromosomes during cell division but do not control cellular activities in general. Chromosomes carry genetic information but are not the structure that controls overall cell activities and undergoes disassembly - reformation during cell division as a whole unit in the way described (chromosome structure changes but not in the same disassembly - reformation as the nuclear membrane).

Answer:

Nuclear membrane