Final answer:
The property in the CSS rule '.error_text { color: red; }' is 'color', which specifies that the text color should be changed for elements with the class .error_text.
Step-by-step explanation:
In the given rule .error_text { color: red; } within CSS (Cascading Style Sheets), the property is color. This is because, in CSS, the property is the aspect of the element that you want to style; in this case, you are indicating that you want to change the text color.
The value 'red' specifies the desired effect on the property, which is setting the text color to red for elements with the class .error_text.
In cascading style sheets (CSS), properties define the visual presentation of HTML elements. Here, the property "color" is associated with the class selector ".error_text," indicating that text within elements having this class should be displayed in the color red.
Understanding CSS properties is fundamental for web development, allowing designers to control and customize the appearance of various elements on a webpage, contributing to a visually cohesive and aesthetically pleasing user experience across different platforms and devices.