Which part of speech is the underlined word in the following sentence? Understanding this process is crucial.

The word 'this' in 'this process' specifies which process is being referred to. Words that specify or point to nouns (this, that, these, those, the, a/an, some, etc.) are determiners. It is a demonstrative determiner here, modifying the noun 'process'. Hence option (1), Determiner, is correct.