I believe the answer is: B) Temperature.
This is due to both Laws tend to revolve around temperature, Charles's Law tends to relate that gas tends to expand upon warming up (from what I make of it), whereas Boyle's Law talks about how the pressure and volume of gas have an inverse relationship when the temperature remains the same.
Both seem to state that if the temperature changes or stays consistent that the gas seems to change or is affected in some way.
I hope this helps, have a great rest of your day! ^ ^
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