Hello!
13.
The radii are the lines that go from the exact center of the circle to anywhere on the circumference of the circle.
Here we can see that segments AB AD and AC go from the center to somewhere on the circumference, thus these segments are radii.
14.
Since the triangles are congruent, their corresponding angles are congruent. Given this, angle A is congruent to angle D and angle C is congruent to angle F.
A tip for future problems like this - you don't necessarily need to look at the diagram; the congruent statement gives you all the information you need about which angles correspond with each other and thus are congruent. Triangle ABC is congruent to triangle DEF: A corresponds to D, B corresponds to E, and C corresponds to F. This is useful because sometimes the triangles in the diagrams are not oriented the same way and its confusing to try to figure out which angles correspond.
15.
The diameter of a circle is the is a line that goes from a side through the center and to the other side of a circle.
The diameter is segment RS.
16.
Obtuse = greater than 90 degrees.
Acute = less than 90 degrees
Right = 90 degrees
Straight = 180 degrees (a straight line)
For this angle to be 90 degrees, it would have to be indicated by a little box in the corner; you must never assume an angle is 90 degrees, unless this information is explicitly stated in the question or shown in the diagram. This is not a right angle, nor is it a straight angle (it is not a straight line). That leaves obtuse and acute. Think: Is this greater than or less than 90 degrees? It is less than 90 degrees. The angle is acute.