Here the order is defined for leaf node of B+ tree as :
Maximum no of <key , record ptr > pair in leaf node
So let that p , so we have :
p*(key size + record ptr size) + block ptr size <= 512
==> p*(16) + 6 <= 512
==> p <= 506 / 16
==> p <= 31.625
But we have 32 now it will exceed maximum block capacity which is not allowed.
Hence p = 31 is correct answer..