Concept:Prevention aims to stop diseases from occurring, while identifying disease sources is part of control and investigation.Explanation:Rearing disease-resistant stock prevents infection by strengthening natural immunity.Carrying out rotational grazing reduces the build-up of disease-causing organisms on pasture.Feeding a balanced ration maintains good health and increases resistance to diseases.Identifying the source of diseases happens after an outbreak and is not a preventive measure.Answer:C. identifying the source of diseases