Explanation:

- In the sentence, the word "FUNDAMENTAL" means something basic and essential to the core of a concept or entity.

- Option 1: Productive - This means yielding results or benefits; it doesn't match "fundamental."

- Option 2: Essential - This means absolutely necessary or extremely important, making it a synonym for "fundamental."

- Option 3: Successful - This refers to achieving desired results; it's not related to "fundamental."

- Option 4: Effective - This means producing an intended effect or result; it's different from "fundamental."