Apply $MRTS_{LK}=-\dfrac{\Delta K}{\Delta L}$ between each consecutive pair of combinations, since output is held constant along this single isoquant.

$MRTS_{LK}=-\dfrac{\Delta K}{\Delta L}$ is computed between each consecutive pair of combinations, all lying on the SAME isoquant (output fixed at 100 units):

A to B: $\Delta L = 2-1=1$, $\Delta K = 8-12=-4$. $MRTS_{LK}=-\dfrac{-4}{1}=4$

B to C: $\Delta L = 3-2=1$, $\Delta K = 5-8=-3$. $MRTS_{LK}=-\dfrac{-3}{1}=3$

C to D: $\Delta L = 4-3=1$, $\Delta K = 3-5=-2$. $MRTS_{LK}=-\dfrac{-2}{1}=2$

D to E: $\Delta L = 5-4=1$, $\Delta K = 2-3=-1$. $MRTS_{LK}=-\dfrac{-1}{1}=1$ …

Forgetting the negative sign in $MRTS=-\dfrac{\Delta K}{\Delta L}$ and reporting a negative MRTS is a common error — since capital always FALLS as labour rises along an isoquant, $\Delta K$ is negative, and the leading …