Step 1: Identify the target word and its role. 'Significant' is an adjective modifying 'discovery'. It means important, notable, or having a great effect. Step 2: Test each option as a synonym for 'significant'. 'Minor' = small or unimportant → antonym of significant. 'Major' = large, important, notable → same meaning as significant (synonym). 'Obvious' = easy to see → describes clarity, not importance. 'Recent' = from a short time ago → describes time, not importance. Step 3: Confirm the answer. 'Major' and 'significant' both mean of great importance. Substituting: 'The scientist made a major discovery' — meaning preserved. Answer: major