Answer:
A. PO43-
Explanation:
in the given is buffer . so H2PO4- and HPO42- both are present with equal concentration . Na+ is spectator ion it is also present in the concentration higher than the given species above .
but PO4-3 is not present . so it is lowest concentration
1 km is equal to 100,000 cm

and you need to divide it to 1 km so that you can get rid of the km.
Make note of it and learn from her mistakes :)