Select the most appropriate option to fill in blank number 4.
Show answer & explanation
Correct answer: D
Explanation
The correct answer is Option 4 i.e. Serious
Low levels of literacy are a (4) serious obstacle for economic improvement.
Low levels of literacy are serious issues for any kind of development. Further, 'biggest' will be followed by the article 'the' only.
Hence, Serious is the correct answer.