Answers:

(i)

Set-builder form of A = {x : x ∈ N, x<7}

This set builder form can be explained as x is such that x belongs to the natural number also is less than 7.

(ii) 

Set-builder form of B = {x : x = 1/n, n ∈ N}

This set builder form can be explained as x is such that x =1/n and n belongs to the natural numbers.