One common effect of aging on the liver is:

The choice highlighting decreased repair of damaged liver cells is correct because aging naturally leads to a decline in the liver's regenerative capabilities. The liver is renowned for its ability to regenerate after injury, but as one ages, this regenerative process becomes less efficient. Factors such as decreased cellular proliferation and alterations in growth factor signaling contribute to this diminished repair ability. This decline can make the liver more susceptible to damage from toxins, diseases, or chronic conditions.

In the context of the other choices, while drug metabolism may be expected to decline with age due to changes in liver enzyme activity, it is essential to recognize that some aspects of metabolism do remain intact, and individual variability is significant. Motility in the digestive tract generally does not improve with age, and reduction in bile production occurs, but it is less directly tied to cell repair than the correct choice. These aspects highlight specific nuances in the aging process affecting the liver, reinforcing why decreased repair is a notable effect of aging on this organ.