Final answer:
In Sonnet XIX, the speaker mourns the loss of his eyesight, which is a metaphorical representation of losing the ability to see and perceive beauty.
Step-by-step explanation:
The loss that the speaker mourns in Sonnet XIX is D. the loss of his eyesight. This theme is depicted in the text where the narrator's vision is discussed in relation to the gloom of night and a dimming of the eyes, similar to losing one's sight to darkness. The loss of eyesight is a metaphorical representation of losing the ability to see and perceive beauty, which connects to deeper themes of loss and the passing of time, often lamented in poetry.