The main program synthesizes data and a periodic output-compare interrupt will output the data separated by a fixed time. a fifo queue is used to buffer data between a main program (e. g., main program calls fifo_put).

Your question is incomplete as I've seen the full question before. But anyway, I can help you. The last part of your question looks like this: 
"An output-compare interrupt service routine calls Fifo_Getand actually outputs. Experimental observations show this FIFO is usually empty and has almost three elements."
And you need to answer what it means.
So the answer is "the system is CPU bound". It means that the program is bottlenecked by the CPU.

The answer would be d (tables)