This sonnet is most likely about the difficulty of aging. The speaker reflects on the physical changes and challenges that come with aging, such as knee pain and arthritis. However, despite these challenges, the speaker finds a sense of youthfulness and vitality through the love and affection of their partner, who still loves them just as much as they did when they first fell for each other.
The sonnet mentions a number of physical changes and challenges associated with aging, including "ageing eyes," "knees" that "protest" when the speaker tries to get up from a chair, and "arthritic fingers" and a "problematic neck" that cause pain. The speaker also notes that seeing a photograph can be a reminder of the "dismal truth" of having left their youth behind.
Overall, the sonnet explores the theme of aging and the ways in which love and affection can help a person feel young again, even in the face of physical challenges and changes. Therefore, the answer is option a: The difficulty of aging.