Concept:Day-old chicks cluster near heat when the brooder temperature is lower than their required comfort level.
Explanation:Young chicks cannot regulate their body temperature well during the first few days after hatching.
They depend on the brooder to provide steady warmth.
When the environmental temperature is too low, they naturally move closer to the heat source to stay warm.
This behaviour is a normal thermoregulatory response, not a sign of sickness.
It is also not caused by thirst or poor feeding, because chicks show different signs for those problems.
Therefore, the observation indicates that the brooder house is experiencing low temperature.
Answer:D. low temperature