In what setting do all frequencies of electromagnetic waves travel at the same speed?

You're fishing for "vacuum", but the question shows some uncertainty
with the concepts.

All electromagnetic waves travel at the same speed through vacuum.
That's a fact.

It's also true that they all travel at the same speed through jello, but it's
slower than their speed in vacuum.

The key point is that all electromagnetic waves travel at the same speed,
as long as you're measuring them all in the same material medium.

The speed is highest in vacuum, and some other different, lower speed
in different materials.

Amethod of procedure that has characterized natural science since the 17th century, consisting in systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses.

Good moring let me you on this question the answer the electric force  have a good day ; )