In the word "MODELS," the letters are M, O, D, E, L, and S. Among these letters, the vowels are O and E.
To find the probability of selecting a vowel letter, we count the total number of vowels (2) and divide it by the total number of letters (6).
The probability \( P \) of selecting a vowel is given by:
\[ P(\text{vowel}) = \frac{\text{number of vowels}}{\text{total number of letters}} = \frac{2}{6} = \frac{1}{3} \]
Therefore, the correct response is:
\(\frac{1}{3}\)