Answer : 4. "Covalent bonds"
Explanation :

Answer: D) Covalent bonds Explanation: Deoxyribose, which is a pentose, and a phosphate group are the two molecules together form the two sides of the DNA i.e, A sugar (deoxyribose) and a phosphate. DNA is a double-stranded molecule twisted into a helix (think of a spiral staircase). Each spiraling strand, comprised of a sugar-phosphate backbone and attached bases, is connected to a complementary strand by non-covalent hydrogen bonding between paired bases. The bases are adenine (A), thymine (T), cytosine (C) and guanine (G). A and T are connected by two hydrogen bonds. G and C are connected by three hydrogen bonds.