What Does It Mean to Say That a Set of Obligations

What does it mean to say that a set of obligations is unorderable?

A) That for each obligation in the set, there is some situation in which it overrides each of the others and some situation in which each of the others overrides it.
B) That each of the obligations can be reduced to some more basic obligation, but that none of the obligations can be reduced to the same obligation.
C) That none of the obligations can be reduced to any other obligation, including any of the other obligations in the set.
D) That each of the obligations in the set can only be known through careful reflection on the meaning of the words used to state the obligation, rather than as the conclusion of an argument.