"Which Shinkansen is the fastest?"
どの新幹線 "が" 一番速いですか？
I can't write in English very well, so I explain in Japanese. Excuse me.
You can usually replace が (and any other particle) with は, even though it's not a topic, but we can't in this case.
Question words like どの/どれ/だれ/どこ are special and always use the particle が.
A reason why this is is because は moves emphasis away from itself (also why は clauses are often omitted), while が emphasizes itself. But question works like どの are asking specifically "which?," and wants emphasis. If you put は and どの together, the emphasis will clash, but が and どの both want emphasis, so they work well together.
The translation of いちばんはやいしんかんせんはどれですか？should be "Which is the fastest Shinkansen?
That's what I thought. As others have noted, what duo actually wants you to write is "which is the fastest shinkansen?"
I answered どれの新幹線は一番早いですか and it corrected 早→速. On the next round, after making that correction, it told me the correct answer should be いちばんはやいしんかんせんはどれですか