Where reference is made to this Article, passengers shall be offered the choice between:
(b) re-routing, under comparable transport conditions, to their final destination at the earliest opportunity
Now, Article 8 is referenced from: Article 4 Denied boarding, Article 5 Cancellation, in Article 6 Delay we have "when the delay is at least five hours, the assistance specified in Article 8(1)(a)" -- but that's not (b).
I can't for my life figure out where does EU 261 mandate such a thing. There is something because the CAA has this:
If you still want to fly then your airline must get you to your destination. You might have to be patient while they rearrange flights and rebook passengers, but the law says they must get you there.
But it's not clear under what circumstances does that kick in. The Your rights when you miss a connection only talks about compensation and not re-routing.