A DNA contain 30,000 base pairs. How many nucleotides would be present in it?

The correct option is D 60,000
In DNA molecule, there are two complimentary stands wound as a double helix. Each strand is composed of repeating units of nucleotides (composed of a deoxyribose sugar, nitrogenous base and a phosphate group) joined by phosphodiester bonds.
The nitrogenous base of each nucelotide pairs up (via hydrogen bonds) with the complementary (A with T and G with C) nitrogenous base of the nucleotide on the opposite strand. Thus each base pair involves two nucleotides. So 30,000 base pairs will have 60,000 nucleotides.