Correct option is A
In a submerged rectangular weir, aeration of the nappe is essential to:
Prevent formation of partial vacuum beneath the nappe.
Ensure that the pressure under the nappe remains at atmospheric pressure.
This condition maintains a stable and predictable head–discharge relationship, which is independent of time.
Without proper aeration, fluctuating pressures develop below the nappe, causing unstable discharge measurements and unreliable results.