Food services are available 24 hrs a day.
Food services are available for 24 hrs a day.
Which one is correct?
Both are correct: it’s perfectly acceptable to drop the “for,” making “24 hrs. a day” an adverbial noun and getting rid of the prepositional phrase. Generally speaking, the shorter sentence is always preferred. (But using the prepositional phrase instead doesn’t sound too unnatural.)
The inclusion of the word for would seem to imply that there is a set duration on the availability, for example, in the following phrase:
Food services are available for 2 hours on Monday.
Whereas, since the time period in your example conveys that the food services are continually available, the word for is better omitted.