Any angle can be named by a single letter, If the letter represents its vertex.
In the given angle RST,
S is the vertex.
1 is the name given to angle.
So we can name the angle in the following ways
<RST, <TSR, <1, <S.
Here R is a point on one of the arms of the angle and is not a vertex.
Hence we cannot name the angle as <R.
All of the following are acceptable ways to name the angle shown except <R.