What holds the double strands of DNA together?

covalent bonds link the strands to each other
hydrogen bonds hold the base pairs together
ionic bonds keep the deoxyribose linked to the phosphate groups

Correct! The double strands of DNA are held together by hydrogen bonds between the base pairs, while covalent bonds link the strands to each other. Additionally, ionic bonds help keep the deoxyribose sugar linked to the phosphate groups in the backbone of the DNA molecule.