A unit rate is a rate that compares a quantity to one unit of another quantity. For example, if you want to determine how many units of one item are contained in a single unit of another item, you would express it as a fraction where the denominator is 1.
Among the options provided, only \( \frac{1}{27} \) is a unit rate, as it indicates a quantity of 1 compared to 27 of another unit.
Therefore, the correct answer is:
\( \frac{1}{27} \)