The third generation vaccine-DNA vaccine refers to the recombinant DNA molecule obtained by inserting the gene encoding antigen protein into a suitable plasmid and introducing it into human body.

A, DNA vaccine is a gene encoding antigen protein, so its expression product is antigen, and A is wrong;

B, DNA vaccine is a recombinant DNA molecule, its basic unit is deoxynucleotide, and B is correct;

C, DNA vaccine is a gene encoding antigen protein, not a modified hepatitis B virus, and C is wrong;

D, DNA vaccine refers to the recombinant DNA molecule obtained by inserting the gene encoding antigen protein into a suitable plasmid, indicating that DNA ligase and restriction endonuclease are needed in the production process, and D is wrong.

Therefore, choose: B.