Answer: Option D.
Cyclins are proteins that regulate the cell cycle at molecular level.
Step-by-step explanation:
Cell cycle refers to series of activities that take place in the cell leading to replication of DNA and division of cytoplasm to give two daughter cells. Cell cycle are regulated by a set of proteins called cyclins.
Cyclins are group of proteins the bind and activates cycling dependent kinases enzymes in order to control cell progression through the cell cycle.