Final answer:
In a normal frequency distribution of letters in the English language, the letter 'e' is encountered more often than any other letter. The correct option is 'c. E'.
Step-by-step explanation:
When conducting a normal frequency distribution of letters, the letter that is more often encountered in the English language is 'e'. Letters in the English alphabet have different frequencies of occurrence, with 'e' being the most common letter used in texts such as books, articles, and general writing. This can be attributed to the phonetic and linguistic structure of English, where 'e' is a vowel that appears frequently in many commonly used words.
In contrast, the letter 'z' is one of the least used letters in English, making it unlikely to be the answer to this question. Similarly, 'a' and 'o' are vowels and are used more frequently than 'z', but still less commonly than 'e'. Thus, considering the typical distribution of letter usage in English texts, 'c' is not the correct answer either.
Therefore, among the given options (a. A, b. Z, c. E, d. O), the correct option is 'c. E'.