Which part of speech is the underlined word in the following sentence? On the whole that is true.

In the sentence 'On the whole that is true', the word 'that' is used in place of a noun/idea (referring back to a previous statement). It functions as a pronoun (demonstrative pronoun). It does not modify a noun (determiner), nor does it link clauses (conjunction), nor show relation (preposition). Hence option (2), Pronoun, is correct.