Mangrove pneumatophore roots
a.
prop the trees up above the water line
b.
excrete excess salt
c.
absorb carbon dioxide from the air
d.
none of the above

Correct A). Prop the trees up above the water line

Mangrove pneumatophore is the specialized root structures that are known to grow out of the water surface and help in the respiration of mangrove plants. They allow the collect oxygen from the atmosphere.

It mainly grows in mud and there the conditions are anaerobic means lack of oxygen. Oxygen is necessary for their survival that is done by the mangrove pneumatophore roots.  

Hence, the correct answer would be option A.