The hydrophilic nature of DNA is due to the presence of the phosphate group. DNA is made up of nucleotides, which consist of a phosphate group, a sugar molecule (deoxyribose sugar), and a nitrogenous base. The phosphate group has a negative charge and is attracted to water molecules, making it hydrophilic. This property allows DNA to dissolve and be transported in the watery environment of the cell.

Option 1, thymine base, is incorrect because the specific nitrogenous base does not contribute to the hydrophilic nature of DNA.

Option 2, a number of hydrogen bonds, is also incorrect because although hydrogen bonds are present between the nitrogenous bases in DNA, they do not directly contribute to the hydrophilic nature of DNA.

Option 4, deoxyribose sugar, is incorrect because the sugar molecule also does not contribute to the hydrophilic nature of DNA.

So, the correct answer is option 3, the phosphate group.