Hey there! I'm happy to help!
Functional illiteracy means that you cannot function well in life because you cannot read or write. A first, sixth, or even tenth grader can be illiterate but still be able to function in life because they could be a be playing video games all day, watching videos, etc. all day but they are still able to function because their parents work to meet the needs of their kid. But when you are in twelfth, you will be an adult very soon and you have to be off on your own, and literacy is crucial here. If you cannot read or write when you are 18, it will be extremely hard to function at all.
Therefore, the answer is D. Twelfth.
Have a wonderful day! :D
