The answer is (2) hydrogen bonding.
The double helical structure of DNA is due to the hydrogen bonds between the nitrogenous bases. The two strands of DNA are held together by hydrogen bonds between the nitrogenous bases on each strand. Adenine (A) always pairs with thymine (T) with two hydrogen bonds, and cytosine (C) always pairs with guanine (G) with three hydrogen bonds.
Van der Waals forces, dipole–dipole moment, and electrostatic attractions are all weaker forces than hydrogen bonds, so they are not strong enough to hold the two strands of DNA together in a double helix.
Therefore, the only option that is the reason for the double helical structure of DNA is hydrogen bonding.
The other options are incorrect. Option (1) van der Waals forces, are weak attractive forces between molecules that are caused by the temporary uneven distribution of electrons. Option (3) dipole–dipole moment, is a measure of the polarity of a molecule. Option (4) electrostatic attractions, are the attractive forces between oppositely charged ions. | 
