Sentence:  The D minor Sonata , Op. 31 No. 2 , introduced by dynamically shaped arpeggios , <WSD>was</WSD> most engaging in its moments of quasi-recitative - single lines in which the fingers seemed to be feeling their way toward the idea to come . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)