How are timing assignments set?

What you're seeing is the combinatory propagation delay from "b" to "c", through the logic that implements it.

There's no way to make it instantaneous; but sometimes, it can be made faster.

You can try to set the synthesis and fitter options set to "speed", instead of area or balanced.

You can also try to use a more modern version of Quartus and use TimeQuestl, instead of the Classic Timing Analyzer.

And of course, you can try to figure out why the combinatory path is so long and see if there's a shorter way.