Final answer:
The line "Here in this alien crowd I walk apart / Clasping remembered beauty to my heart!" best reflects the theme of profound emotional experiences capable of touching a heart of stone.
Step-by-step explanation:
From the provided selections and the themes of emotions such as joy, grief, and strong feelings amidst memories, the line that best supports the theme "once I have known / The joy that could have touched a heart of stone" is c) "Here in this alien crowd I walk apart / Clasping remembered beauty to my heart!". This line encapsulates the idea of clinging to a poignant memory that holds the power to move one deeply, akin to the theme of an experience so profound it could affect even the hardest of hearts.