ANSWER
The segment that is a radius is
[tex]\bar{RO} [/tex]
EXPLANATION
A radius is any line segment from the center of a circle to the circumference.
From the above diagram, the center of the circle is O.
The points on the circumference are A, R S and T.
The only line from the circumference to the centre in the options above is the line segment RO.
The correct answer is B.
Note however that
[tex]\bar{AS}[/tex]
is a diameter, which is a special chord.
[tex]\bar{RA}[/tex]
is a chord.
[tex]
\bar{ST}[/tex]
is also a chord.
