Which value for the 'display' property will make an element act like a block element?

The value for the 'display' property that makes an element act like a block element is indeed "block." When an element is set to display as block, it takes up the full width available by default, and any subsequent elements will be placed on a new line beneath it. This is a fundamental aspect of how block elements behave in the box model of CSS.

Block elements, such as <div>, <h1>, <p>, etc., stack vertically in the layout, and their width can be controlled by adjusting margins and padding. This behavior distinguishes block elements from inline elements, which only take up as much width as necessary and do not force a line break.

In contrast, the other options present different display behaviors. For instance, "inline" allows the element to flow with surrounding text without creating line breaks, while "flex" establishes a flex formatting context, enabling more complex layouts that can rearrange child elements. "Inline-block" similarly offers an inline flow but allows for setting width and height, unlike a traditional inline element. Understanding these distinctions is key to controlling layouts with CSS effectively.