Hv Ziwei logo is too big.

A. According to the formula of tag recapture method: the number of individuals in the population (N)/ the total number of tags = the total number of recaptured individuals/the number of tagged individuals in recapture, we can know that if some tags on magpies fall off, the number of tagged individuals in recapture will be less, and the final experimental value will be greater than the actual value, so A is wrong;

B, sampling method When investigating the population density of Taraxacum mongolicum, it should be randomly sampled, and sampling in densely distributed areas will be too large, and B is wrong;

C, normal statistics, the cells on the edge of the border of the blood cell counting board should be counted on two adjacent sides, so only the cells in the counting room box are counted, and the experimental value is less than the actual value, so c is correct;

D if the incidence of genetic diseases is investigated, samples should be taken from the population. If the patient's family is taken as the object of investigation, the value obtained will be greater than the actual value, and D is wrong.

So choose: C.