To answer this question, you need to understand how attribute values are represented in HTML or XML.
Option A) Parenthesis - This option is incorrect because attribute values are not enclosed by parenthesis in HTML or XML.
Option B) 'Greater-than and less-than signs' - This option is incorrect because greater-than and less-than signs (< and >) are used to enclose HTML or XML tags, not attribute values.
Option C) Single or double quotes - This option is correct. Attribute values in HTML or XML can be enclosed by either single quotes ('') or double quotes (""). For example, <img src="image.jpg"> or <p class="text">.
Option D) Brackets - This option is incorrect because brackets are not used to enclose attribute values in HTML or XML.
The correct answer is Option C. Attribute values must always be enclosed by single or double quotes.