In CSS, what is a selector and why is it important in defining styles for an HTML document?

Introduction / Context:
CSS is all about selecting the right elements and then applying styles to them. The term selector refers to the part of a CSS rule that decides which elements are affected. Without selectors, style rules would have no clear targets and could not change the appearance of specific parts of the page. This question checks if you understand what a selector is and why it is central to how CSS works.

Given Data / Assumptions:
- We are using CSS to style HTML documents viewed in a browser.
- A CSS rule contains at least one selector and a set of declarations.
- The goal is to attach visual properties to selected HTML elements.
- Selectors can refer to element names, classes, ids, attributes, and more.

Concept / Approach:
In a CSS ruleset, the selector appears before the curly braces, and the declarations appear inside. The selector can be as simple as a tag name, such as p, or more complex, such as .highlight or #main. When the browser parses CSS, it uses the selector to find matching elements in the document tree. It then applies the specified property value pairs to those elements. The correct option must describe selectors as patterns that choose elements for styling, not as server components or database tools.

Step-by-Step Solution:
Step 1: Recall the basic CSS syntax where selector is followed by curly braces and declarations. Step 2: Think of examples like p, .className, or #header, which clearly act as selectors. Step 3: Review each option and look for the statement that describes selectors as patterns used to target HTML elements. Step 4: Choose the option that matches the definition from CSS documentation and practice.

Verification / Alternative check:
You can verify this understanding by writing a small CSS example such as p { color: blue; }. Here, p is the selector and the browser colors all paragraph elements blue. Changing the selector to .note will only affect elements with class note. This experiment confirms that selectors are patterns that identify elements for styling.

Why Other Options Are Wrong:
Option B is incorrect because database row selection is unrelated to CSS selectors. Option C is wrong since server host names do not appear in CSS rules. Option D is false because file extensions such as .css name files, not selectors. Option E is unrelated to CSS and refers to operating system tools instead of web styling.

Common Pitfalls:
A common mistake is writing overly broad selectors that style more elements than intended, leading to hard to debug layouts. Another pitfall is using very specific selectors that make styles difficult to override. Developers should learn to choose selectors that are clear, maintainable, and reflect the semantic structure of the HTML document.

Final Answer:
A CSS selector is a pattern that tells the browser which HTML elements to target so that the style declarations in the rule are applied to those elements.