Think back to earlier in this chapter: intermediate III could form from isomerization of the carbonyl group in compound B.
How many carbon atoms are present in the reactant and product? Reactivity on the molecule refers to the location of reactive atoms or functional groups. Working backwards, we realize that the beta-methoxy group could be formed from beta-hydroxy group by a SAM methylation reaction.
Now, starting with the R1 group and working along the carbon chain, we can account for carbons a-e on the two precursors.
For now, we'll stick with our initial choice to make the methylation step the last one. Therefore, we can make a likely disconnection next to the alpha-carbon in the target molecule.
You do, however, have right now enough of a bioorganic repertoire to begin to learn how multi-step pathway problems can be approached, using for practice some generalized, hypothetical examples in which the reaction types involved are limited to those with which you are already familiar.
This is why it is important to review past topics prior to moving on to the next chapter. Both reactant and product have three carbon atoms. What I always found useful was to learn which reactions work the best for this class.
We have a single functional group in the reactant and product.