Final answer:
The character & is not considered a word character, as word characters usually include letters, numbers, and the underscore.
Step-by-step explanation:
To answer your question, the character that is not a word character is d. &. In typical definitions used by regular expressions and programming languages, word characters include uppercase and lowercase letters, numbers, and the underscore (_). Therefore, a. w, b. ₀ (a subscript zero), and c. _ (underscore) are all considered to be word characters. The ampersand (&) is not considered a word character because it is a punctuation symbol used for logical operations and entity references in various languages.