The correct answer is option 'D', her declining age.

Explanation:
When the poet looked at her mother, she was worried about her declining age. The poem portrays the emotions and concerns of the poet as she observes her mother getting older.

1. Concern for Aging:
The poet's worry stems from the realization that her mother is growing older and that time is taking a toll on her. This is evident in lines such as "the mother's smile has a tinge of sadness" and "her eyes have lost their luster." The poet is deeply affected by her mother's aging appearance, which causes her worry and concern.

2. Emotional Impact:
The poet's concern for her mother's declining age is emphasized through the emotional impact it has on her. The poem describes the poet's heart sinking as she looks at her mother, implying a sense of sadness and anxiety. The poet's worry is a reflection of her deep love and attachment to her mother.

3. Fragility and Vulnerability:
The poet's worry also arises from the realization of her mother's increasing fragility and vulnerability. The line "the bones beneath the skin are so fragile" indicates that the physical strength of her mother is diminishing. This vulnerability adds to the poet's concern and apprehension.

4. Time's Influence:
The poem highlights the inevitable passage of time and its effect on the poet's mother. The poet becomes acutely aware of the fleeting nature of life and the changes that come with age. This realization intensifies her worry and heightens her appreciation for her mother's presence.

In conclusion, the correct answer is 'D', her declining age, because the poet's concern and worry are primarily centered around the visible signs of aging and the emotional impact it has on her. The poem beautifully captures the bittersweet emotions that arise when one observes the aging of a loved one.