Once you've finished this quiz, you'll be more familiar with the future perfect tense.
Not every language probably bothers with this, but it makes useful sense to have a tense that reaches forward to a point in the future and expresses what 'will have been done' by the time that moment arrives. There may well also be an element of the Continuous in it. 'Once you've finished' with this Quiz, you'll be a 'dab hand' at establishing deadlines on your future plans!
Answer 3 is far from bad, but with less of this nuance than No.2; the two outer Answers are also possible, but No.1 assumes an individual who has already reached GCSE (rather than being somewhere along the way towards it), while No.4 suggests ~ in rather formal, even prescriptive tones ~ that a child who has not yet turned 5 years old will definitely 'have it coming to them' whether they fancy it or not!
Incidentally, nobody is pretending that all those 2,000+ hours are actually geared towards two 90-minute Papers as a final target. Maths is far broader and more important than that!