Know the Rule
You are given three diagrams and you have to determine which diagram displays an incident ray that will undergo total internal reflection. There are two requirements; you can read about them in the Fundamentals section (above). The first requirement has to do with the relationship between the angle of incidence and the critical angle. The angle of incidence must be greater than the critical angle. These angles are measured relative to the normal line (the line drawn perpendicular to the boundary.) A protractor is provided; it has markings at 15° increments. Beginning at the normal line, measure the angle out to the incident ray. If it is not greater than the stated critical angle, then you can rule out that diagram.
The second requirement is that the incident light must be in the more dense of the two media. Density information is given for each diagram. Go one by one through the three diagrams and identify the diagrams in which the incident light is in the more dense medium. By the time you have considered these two conditions, there should be only one diagram that has met both requirements for TIR. Select that as your answer.