The simile here clearly indicates this sentiment:
C. A postponed dream can become something difficult to carry.
Explanation:
The sentiment here is between the ability of a load to sag. Any load can only sag when it is being carried. It cannot sag when it is in a state of stasis.
Thus, with this out of the way we can see that the sentence that would explain this simile will have the load being carried by the carrier who sees and notices the sag it has induced.
This is well found in the line where the dream is postponed and thus carried in the conscience of the person that makes it heavy. If the dream is expressed it would not be sagged but it is not.
