acclimatization occurs at the individual level throughout an individual's lifetime.
Hence the correct answer is a.
what is acclimatization?
The process by which an individual organism adapts to a change in its environment (such as a change in altitude, temperature, humidity, photoperiod, or pH) and maintains fitness across a range of environmental conditions is known as acclimatization or acclimatisation (also called acclimation or acclimatation).
Within the lifetime of the organism, acclimatization takes place over a short length of time (hours to weeks) (compared to adaptation, which is evolution, taking place over many generations). This could occur suddenly (like when mountaineers adjust to high altitude over the course of a few hours or days) or it could be a regular occurrence, like when a mammal sheds its thick winter coat in favor of a lighter summer one.
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