Answer is (C).
From the structure of $B+$ tree we can get this equation:
$n\times p + (n-1)\times k \leq B$ ( for non leaf node)
Here, n=order, p=tree/block/index pointer, B=size of block
I non leaf node no record pointer is there in B+ tree.
So, $n\times p + (n-1)k \leq B$
$n\times 6 + (n-1)\times 9 \leq 512$
$\implies n \leq 34.77$
Largest possible value for $n$ is $34.$