You pick the correct sentence from the options. It shows the gap between the grammar you know and the grammar you can actually use.
I have read the grammar book, so why do I still get these wrong?
Knowing a rule and using it are different things. An explanation makes sense while you read it, yet the same point splits you when it sits inside a sentence — that split is what this check looks for.
Which grammar shows up?
Tense, word order, particles and inflection — the things that decide whether a real sentence is right or wrong. It does not test whether you memorised exam-only exceptions.