Everyone of us has to be (1)/ very vigilant to safeguard (2)/ our hardly won liberty. (3)/ No error (4)

Answer

3

Explanation

Hardly (Adverb) = almost no , almost not , none. .
There is hardly any tea left.
Hard (Adverb) : with great effort , with difficulty .
He was still breathing hard after his run.
Hence, our hard won liberty should be used here.