This question is about the importance of adaptive ability in organisms. The correct answer is 'A. Increased vulnerability to diseases' because without the ability to adapt, an organism is less likely to cope with environmental changes and more susceptible to diseases. Option B is incorrect because adaptive ability does not directly enhance reproductive success. Option C is incorrect because adaptive ability would actually help in finding food. Option D is incorrect because while lack of adaptation can increase predation risk, it is not the direct consequence as the increased vulnerability to diseases.