What phenomenon has caused Robbies surprise?

The answer to this is Sensory adaptation. This may not be the answer that you will first think about but the other given choices just do not fit. Sensory overload is the sensation of feeling overwhelmed when the senses are subjected to certain stimulation for a long time. Sensory deprivation is a reduction of some senses in order to promote relaxation.

Transduction is the process of introducing foreign DNA into a cell. This usually happens when cells are exposed to viruses. An absolute threshold is an absolute stimulus that can be detected by the human senses.

Based on these, Sensory adaptation is the best answer because even though he screamed in the beginning, the body will be able to adapt to the freezing water soon enough.