A. After RHOGDl2 gene is added to cancer cells, it is expressed in cancer cells and produces a glycoprotein, thus making cancer cells lose their metastatic ability. A is correct;

B, this gene helps to avoid the spread of cancer cells, and can't make cancer cells quickly differentiate into other tissues and organs. B is wrong;

C, after the gene is expressed in cancer cells, the cancer cells lose the ability of metastasis, but it does not lead to the rapid aging, apoptosis and C error of cancer cells;

D. After the gene is expressed in cancer cells, cancer cells lose the ability of metastasis, but DNA replication will not be blocked, so cancer cells can still proliferate. D is wrong.

So choose: a.