Compute MRSxy = −ΔY/ΔX between each successive pair of combinations.

$A\to B$: $\Delta Y=-4 \Rightarrow MRS=4$. $B\to C$: $\Delta Y=-3 \Rightarrow MRS=3$. $C\to D$: $\Delta Y=-2 \Rightarrow MRS=2$. $D\to E$: $\Delta Y=-1 \Rightarrow MRS=1$. MRS falls steadily (4,3,2,1 …

Computing $MRS_{xy}=-\dfrac{\Delta Y}{\Delta X}$ for each successive move (each move has $\Delta X=+1$):

Using non-adjacent combinations (say, computing MRS directly between A and C, skipping B) instead of strictly successive pairs gives a blended, less meaningful figure — MRS should …