Hello there!
Your answer would be Emphasis
The principle of design that is used to make one element of a design stand out is the Emphasis principle.
The word "emphasis" means to give special "value" / "importance" to something.
The aligns to what the question is asking because an element of a design needs to be given importance in the design in order to stand out. If they made every element in the design stand out, then none of the elements in the design will pop out. This is where emphasis comes in when people design things. When people design things, they emphasize an element that they really want to have stand out to others, and focus on making that element very important.