Step 1: Understanding the Concept
This question tests the use of indefinite articles ('a' and 'an'). The choice between 'a' and 'an' depends on the sound of the first letter of the word that follows.
Step 2: Key Concept
- Use 'a' before words that begin with a consonant sound. (e.g., a book, a university)
- Use 'an' before words that begin with a vowel sound (a, e, i, o, u). (e.g., an apple, an hour)
Step 3: Detailed Explanation
The word following the blank is "old".
The word "old" starts with the letter 'o', which has a vowel sound.
Therefore, the correct article to use is 'an'.
The sentence becomes: "I met an old friend in the temple."
Step 4: Final Answer
Since "old" begins with a vowel sound, 'an' is the correct article.