Sentence:  Long observation has taught Mason that <WSD>most</WSD> landscape can be reduced to three essential planes : a foreground in sharp focus - either a light area with dark accents or a dark one with lights ; a middle distance often containing the major motif ; and a background , usually a silhouetted form foiled against the sky . 
Meaning of the word marked with <WSD> tags:  the superlative of `much' that can be used with mass nouns and is usually preceded by `the'; a quantifier meaning the greatest in amount or extent or degree