Which is a redox reaction?
a) 2kbr + f2 > 2kf + br2
b) 2hcl + mg(oh)2 --> 2hoh + mgcl2
c) 2nacl + h2so4 --> na2so4 + 2hcl
d) ca(oh)2 + pb(no3)2 --> ca(no3)2 + pb(oh)2

i know the answer is a, but confused as to why that is. can someone explain this for me? .

You have to look at the oxidation states of each atom in the reaction.  Since redox reactions involve the transfer of electrons from one element to another, 2 elements need to have oxidation state changes.  
option a is the only option that has a change in oxidation sates.  Fluorine goes from 0 to -1 indicating that it gained electrons (got reduced) while bromine goes from -1 to 0 indicating it loses electrons (got oxidized).
(2Br⁻→Br₂+2e⁻ and F₂+2e⁻→2F⁻).

I hope this helps.  Let me know if anything is unclear.