Final answer:
The 'center' tag breaks the separation of content and layout principle by controlling the presentation style directly in HTML, while 'p' and 'br' are semantic tags that do not affect layout.
Step-by-step explanation:
The center tag in HTML is the one that breaks the tenet of separation of content and layout. It is because this tag directly influences the layout and presentation of the content by centering text, which is a stylistic choice that should ideally be controlled by CSS (Cascading Style Sheets). The separation of content and layout is a fundamental principle in modern web development that suggests keeping the structure of the web content (HTML) separate from the instructions for how it should look or be presented (CSS).
In contrast, tags like p and br are used for semantic markup. The p tag denotes a paragraph, and br signifies a line break, both of which are related to the content structure rather than its styling or layout. Hence, they are considered to be in line with the principle of separation of content and layout.