The research team used the setup to study the incorporation of labeled nucleotides into a culture of lymphocytes and found that the lymphocytes incorporated the labeled nucleotide at a significantly higher level after a pathogen was introduced into the culture. they concluded that:
a. the presence of the pathogen made the experiment too contaminated to trust the results
b. their tissue culture methods need to be relearned
c. infection causes lymphocytes to divide more rapidly
d. infection causes cell cultures in general to reproduce more rapdily

Option C, Infection causes lymphocytes to divide more rapidly.

Explanation:

Lymphocytes are white blood cells that divide at a fast rate in presence of any antigen, pathogen etc. They become active and attack the foreign antigens only when the innate immunity system is already activated. Here, the labeled nucleotide are found in larger number as compared to the number of labeled nucleotide introduced this means that the rate of division of blood cell was high and this high rate of cell division is due to the pathogen introduction.  

Hence, option C is correct