Explanation

From m : n = 1 : 2 write m = k, n = 2k, and from p : q = 3 : 4 write p = 3r, q = 4r. Then 2m + 4p = 2k + 12r and n + 3q = 2k + 12r are identical expressions. Hence (2m + 4p) : (n + 3q) = 1 : 1, option (a).