✅ Option A is correct. "Whose research changed the field" is a non-restrictive relative clause, providing additional, non-essential information about "the scientist." Non-restrictive clauses are correctly set off by commas, and 'whose' is the appropriate possessive relative pronoun for people (or things) to indicate that the research belongs to the scientist.

❌ Option B is incorrect because the clause "whose research changed the field" is presented as a restrictive clause without commas. Given "The scientist" (singular, definite article), the clause likely provides extra information rather than identifying which specific scientist, thus requiring commas for a non-restrictive clause. If there were multiple scientists and this clause was essential to specify *which one*, then commas would not be used.

❌ Option C is incorrect because 'that' cannot be used to introduce a non-restrictive relative clause (a clause set off by commas). Furthermore, 'that' would not function correctly as a possessive pronoun in this context; it would need to be 'whose'.

❌ Option D is incorrect because 'which' is used for things or animals, not for people in a possessive context, and it also cannot introduce a non-restrictive clause in a possessive manner. 'Whose' is the correct possessive relative pronoun for both people and things.