Does the following also make sense?: "Does the speed of light have to be the fastest speed?"
Yes but it does not match the original sentence in Vietnamese. When posing a question involving this form "is something something", one uses "có phải là". It does not associate with "have to" as the hint suggests.
Thanks a lot!
Is 'phải' necessary here to make a 'có...không' construction using the verb là?
yes. please follow this link for further information.