Which physiological change occurs due to an increase in the hormone oestrogen?

An increase in the hormone oestrogen is primarily associated with the development of secondary sexual characteristics. Oestrogen plays a crucial role during puberty as it drives the development of features such as breast development, the widening of hips, and the maturation of the reproductive system in females. This hormonal change is significant for the onset of menstruation and overall reproductive health.

While other options mention physiological processes, they are not directly tied to the effects of oestrogen. For instance, increased urination can be influenced by other hormones such as antidiuretic hormone, and water absorption in the kidneys is primarily the result of processes regulated by different hormones like aldosterone. Lastly, oestrogen does not cause a reduction in calcium levels; in fact, it often supports the maintenance of bone density and calcium homeostasis. This makes the development of secondary sexual characteristics the most relevant effect of increased oestrogen levels.