The correct answer would be option D, that is, fertilization during the study.
The experimental error refers to the dissimilarity between a measurement and the true value or between the two measured values. Experimental error is determined by its accuracy and precision.
A variation in data, or error, basically signifies whatever results in the data to be imperfect, not the human mistakes. In order to correct this, there is a need to get familiar with the unavoidable sources of error and how they take place so that one can reduce their effect and get the best probable data.