Final answer:
Gregor retains his humanity through an indestructible illusion, which symbolizes his internal strength and moral integrity despite his physical frailty and challenging circumstances.
Step-by-step explanation:
The one item that helps Gregor cling to his humanity is an indestructible illusion.
Throughout the excerpts provided, the theme of clinging to humanity through internal strength and moral integrity is evident. Gregor's character might be struggling with material and external turmoil, but it is his internal power and steadfast hope that preserve his human essence.
Regardless of his dire circumstances, it is hinted that his survival relies on his inner convictions and his ability to cling to a sense of moral righteousness, seen through his contemplation of life's pure and true-hearted aspirations. This intangible but powerful belief is encapsulated in his indestructible illusion which contrasts his physical frailty with a robust internal life and a strong inner morale, acting as a bastion against his struggles letting him see beyond material constraints and circumstances.