The correct option is:
A. Pienso que el chocolate es mejor que la fresa.
This translates to "I think that chocolate is better than strawberry," and uses the appropriate comparative form "mejor" (better).
Options B and C use "bueno" (good) incorrectly in comparative forms - it should be "mejor" for "better." Option D uses "menos bueno" (less good), which is grammatically correct but is not a favorable comparison in this context.