Final answer:
The italicized pronoun 'they' in the given sentence is a plural personal pronoun referring to 'hypotheses'.
Step-by-step explanation:
The italicized pronoun in the sentence 'Scientists test one another's hypotheses to make sure they are sound.' is 'they'. This pronoun is a plural personal pronoun that takes the place of a plural noun (which could be either masculine, feminine, or non-binary) that has already been mentioned or is assumed to be known. In this context, 'they' refers back to the plural noun 'hypotheses', which is understood from the sentence.