There are three primary pigments. Each pigment is designed to absorb a single primary color of light. Cyan pigments absorb red light and reflect blue and green light. Magenta pigments absorb green light and reflect red and blue light. And yellow pigments absorb blue light and reflect red and green light.
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Vocabulary Terms
Let's begin with some vocabulary terms that you should become acquainted with: 

Incident: incident means to approach. Incident light is light that is approaching the shirt. You will need to pay attention to the light that is incident on the shirt.

Pigments: a chemical or dye that is present in a shirt that gives the shirt an ability to absorb specific colors of light. 

Absorb: pigments in shirts absorb or subtract light of a given color. A yellow pigment absorbs blue light. A magenta pigment absorbs green light. In fact, any pigment will absorb its complementary color.

Reflect: reflect means to bounce off. The light color(s) that does not get absorbed by an opaque object will be reflected.  


Using the Incident-Absorbed-Refleted Model
A useful model for analyzing color questions is what we call the incident-absorbed-reflected model. The model suggests that a successful analysis includes three steps ... with each step involving the answering of a question. Those steps or questions are ...

1. What primary color(s) of light are incident upon the shirt?
2. Of the primary light colors that are incident, which color(s) is/are absorbed by the shirt?
3. Of the primary light colors that are incident, which color(s) is/are reflected by the shirt?

The abosrbed light color(s) is/are subtracted from the incident light colors. Once the subtraction process is complete, the result is the reflected light color. It is the reflected light color that reaches the observer's eye and determines the color appearance of the shirt.

In the question provided as an example above, red, green, and blue light are incident upon the shirt. The shirt contains magenta and yellow pigments. Magenta pigments will absorb green light. Yellow pigments will absorb blue light. So of the three primary colors of light incident upon the filter, two of them are absorbed. Only the red light is not absorbed. Since red light is not absorbed, it is reflected towards the observer's eye. As such, the shirt will appear red.

A useful representation used by the incident-absorbed-reflected model is a color subtraction equation. The color subtraction equation can be written as ...
 

RGB - G - B = R

Your question may not be exactly the same as the example that is given here. But the principle remains the same. Apply the model to your question and come up with the answer.