A, subculture of animal cells generally to 10 generation or so will appear growth stagnation, a is correct;

B, to extract ginsenoside, the medicinal component in ginseng, it is necessary to culture ginseng tissue to callus stage, and B is wrong;

C, in the process of cultivating Chinese cabbage-cabbage, the cell wall should be removed first, then the protoplast fusion of the two should be induced by PEG, and then the experimental purpose can be achieved through plant tissue culture, C error;

D, when using biological missiles to kill cancer cells in situ, monoclonal antibodies linked to chemical drugs play an accurate role in positioning, but they can not greatly enhance the killing ability of drugs to cancer cells, nor can they kill cancer cells on a large scale, D error.

So choose: a.