2016 AMC 12A Problem 14

Each vertex belongs to $3$ faces, so $6S=3(1+2+\cdots+8)=108,$ giving each face-sum $S=18.$ The four-element subsets containing $1$ with sum $18$ are $\{1,2,7,8\},$ $\{1,3,6,8\},$ $\{1,4,5,8\},$ and $\{1,4,6,7\}.$ Only one omits $8,$ so at least two of the three distinct faces through $1$ contain $8.$ Two vertices of a cube lie on two common faces exactly when they are adjacent; hence $1$ and $8$ are adjacent. Rotate the cube so that $1$ is at the lower-left-front vertex and $8$ at the lower-right-front vertex. The unique face through $1$ that does not contain $8$ must use $4,6,7,$ and these can be placed in $3!=6$ orders. Each order forces $5,3,2$ at the three opposite vertices, and the remaining face sums are then $18.$ Hence there are $6$ arrangements. Thus, the correct answer is C.