Which code snippet will select all HTML elements with the class "alert" and set their color to red?

The choice that successfully selects all HTML elements with the class "alert" and changes their color to red is the one that starts with .alert. In CSS, a class selector is denoted by a period (.) followed by the class name.

When you use .alert { color: red; }, you are instructing the browser to apply the style specified (in this case, color red) to every HTML element that has the class "alert". This is the standard way to target classes in CSS.

The other options do not correctly follow the CSS syntax for selecting classes. For instance, .alert:hover refers specifically to the state of the element when it is hovered over, not a general style for all elements with that class. The third option, class.alert, is incorrect because it uses an invalid selector syntax; CSS does not start class declarations with the word "class." Lastly, alert { color: red; } would mistakenly look for an HTML element named "alert" rather than selecting elements by the class name.

Understanding these nuances in CSS helps in effectively applying styles across your web pages.