To express the number 18 as a fraction in simplest form, we can write it as 18/1. This is because any whole number can be represented as a fraction by putting it over 1.
To verify if this fraction is in its simplest form, we need to check if the numerator (18) and the denominator (1) have any common factors other than 1. Since 1 does not divide any number other than itself, the greatest common divisor (GCD) of 18 and 1 is 1.
Thus, 18/1 is already in its simplest form. Therefore, when asked to write 18 as a fraction, the simplest representation is 18/1.