Microtubule subunits (for spindle fibers) are synthesized in _ phase.

Explanation

Microtubule subunits are tubulin proteins, which are formed prior to the initiation of mitosis. They are formed during the G2 phase, hence option C is the correct answer. During the G2 phase, the necessary microtubule subunits, which are α- and β-tubulin proteins, are synthesized and then assembled into microtubules to prepare for cell division. The cell increases its mass and produces molecules, including microtubule components, needed for the upcoming mitosis. These newly formed microtubules are then organized into specialized structures like the preprophase band in plant cells or contribute to the growing spindle apparatus that will separate chromosomes during the M phase. G1 phase, involves normal growth and protein synthesis of the cell, leading up to the DNA replication phase (S).M is mitosis in which nuclear division occurs and it consists of prophase, metaphase, anaphase, and telophase. Prophase involves the assembly of microtubules into the spindle fibers, but the subunits of the microtubules, which are tubulin proteins, are formed prior to this stage, i.e. G2.S phase involves DNA replication.