Question

which is more reactive aluminum or magnesium?

Answer 1

if you have a periodic table with you, the further you go down a GROUP, the more reactive the substance.

Answer 2

what the person above mentioned is correct, but not applicable to the question posed because Al and Mg are on the same row.

But you can ask yourself, which has more electrons to get rid off? If this is on a test know that more electrons to remove = more energy needs to be used and thus less reactive.. generally... To be 100% sure check a reduction potential table.