The quotation that indicates people have a fragile hold on life is:
C. Unconverted men walk over the pit of hell on a rotten covering, and there are innumerable places in this covering so weak that they won't bear their weight, and these places are not seen.
This metaphor suggests that people's lives are precarious and that they are in constant danger, as the "rotten covering" symbolizes a weak foundation that could collapse at any moment.