Final answer:
The best word to complete the sentence about Isherwood's attitude towards the working class during the interwar period is 'proclivity,' indicating his natural inclination to engage with them as equals.
Step-by-step explanation:
Isherwood's sympathy for communism during the interwar period was a complex stance influenced by his opposition to fascism and his solidarity with the working class. His attitude suggests a willingness to engage authentically with working people. Therefore, the word that best fills in the blank in the statement is proclivity, which means a natural inclination or predisposition towards something. The complete sentence would be: Isherwood's sympathy for communism during the interwar period was not only a reaction against fascism, but also a mark of his fellow feeling for the laboring classes and his proclivity to engage as an equal with working people.