D. 511
Logic is how much can we push 9th min element towards the bottom of the min_heap ? it is upto the $9$th level.
If we assume numbers to be $1,2,3,4,5...1000$ Then following is a possible arrangement.
Put $9$th minimum in the extreme right corner of the heap in the $9$th level. Remaining nodes we can fill up in ascending order from the unused numbers.
For example