Answer:
<R
Explanation:
We can name the angle by the 3 letters on the rays with the letter on the vertex in the middle
<RST or <TSR
We can name the angle by the letter in the middle if there are no other angles using the vertex
<S
We can name it with a number (or letter) on the inside of the angle
<1
We cannot name it angle R because the point R is not the vertex of an angle