Final answer:
The blank in the sentence can be filled with a comparative adjective like 'better' or 'fastest', to indicate John's coding abilities relative to the other website developer.
Step-by-step explanation:
The sentence 'John is the ________ coder of the two website developers.' is a comparison structure in English grammar. It is typically constructed using comparative adjectives. In this case, it could be completed with comparative adjectives such as 'better' or 'fastest'. For instance, if John is more proficient than the other website developer, you could say, 'John is the better coder of the two website developers.' If John is not only better than the other developer but the quickest among any that you might compare him with, you could say, 'John is the fastest coder of the two website developers.' Thus, the blank should be filled with a comparative adjective that accurately describes John's coding abilities relative to the other website developer.
Learn more about Comparative Adjectives