The base pairs in a dna strand are held together with hydrogen bonds. in what way would a cell be most affected if these were covalent bonds instead? a. the cell would need to make new dna every time the strand was open b. the cell would be able to open and close the dna strand when it replicated c. the cell would have to take in and use in order to brake the covalent bonds d. the would need to replace the nutritious bases each time the dna strand opened.

The answer (although it appears to be missing a word or two) is:
C) The cell would have to take in and use (energy, resources) in order to break the covalent bonds. 

Covalent bonds are more difficult to break/create than hydrogen bonds, and thus require more energy. The advantage of hydrogen bonds to connect base pairs in a DNA strand is the ease with which they can be broken and then connect again, sort of like a zipper. The same process with covalent bonds would be more like unbuttoning and re-buttoning a shirt, it is slower and requires more energy to accomplish.