D is the right one.
The DNA unravels to make two strands
A DNA molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits (A, G, C and T). Each of these chains is known as a DNA strand. Hydrogen bonds between the base portions of the nucleotides hold the two chains together.
Before a cell divides, its DNA needs to be replicated or duplicated in order to ensure that each new cell receives the correct number of chromosomes. This process of DNA replication occurs in the S-phase of interphase stage during cell cycle.
Before this DNA replication can occur, the length of the DNA double helix about to be copied must be unwound into two single strands. The two strands become separated by breaking the weak hydrogen bonds that link the paired complementary bases (A-T, G-C). This process of unzipping the double-helical structure into a Y-shape known as replication fork is carried out by DNA HELICASE. The Y-shaped replication fork will be the template for replication to begin.