Answer:
hydrogen bonds
Step-by-step explanation:
The nucleotides in a strand of DNA are held together by phosphodiester bonds (a specific type of covalent bond). The two strands of DNA are held together by hydrogen bonds that form between the nitrogenous bases in one strand and the nitrogenous bases in the other strand.
Those are the two types of bonds you will probably be required to know for a basic biology class.