yes answer should be C.. what will D help in? recollaring is important to track their movements. but if rangers are using tranquilizers unnecessarily or for reasons other than re collaring rhino's then we need to look into that matter.

D is actually out of scope.. since we are not talking about male rhino's here. Further if C is evaluated and a conclusion is reached then it will cause the argument to get strengthened or weakened. say if its found that rangers are giving tranquilizers for other reasons then the argument falls because we are comparing frequently re collared Female Rhino's to un collared female rhino's.
frequently tranquilized rhino's for other reasons is not mentioned, hence will ruin our entire argument.