One (1) is

When a rational number is multiplied by 1 the number obtained will be the same number,hence one (1) is the identity for addition of rational numbers.

(a)Incorrect because when 1 is added to a number the result obtained will not be the same number to which 1 was added.

(b) Incorrect because when 1 is subtracted from a number the result obtained will not be the same number.

(d)When a number is divided by 1 the number will be the reciprocal of the number and will not have the same value.