Final answer:
The upper critical temperature for a feedlot steer, where it can maintain normal body temperature without additional cooling, is approximately 30°C (option b).
Step-by-step explanation:
The upper critical temperature for a feedlot steer, which represents the highest environmental temperature at which the animal can maintain its normal body temperature without needing to expend extra energy to cool itself, is generally about 30°C. If the temperature rises above this threshold, the steer may begin to experience heat stress, leading to decreased feeding, weight gain, and overall health. The options provided suggest that 24°C is fairly comfortable, 35°C is likely too hot, and 40°C is certainly too extreme for the steer's well-being, making 30°C the most reasonable answer (option b).