I am assuming there was a question (e.g. what fraction of this shape is shaded?).
Ignoring the letters there are three possible numerical answers: 1, ½ and ¼. Only one of them can be right.
I'm not sure we can read anything into the value ¼ being listed twice. The question setter may just have been feeling mischievous. That suggests the chance of guessing the right numerical answer is 1 in 3.
But with multiple choice questions which have 4 choices the answer has to be a or b or c or d. You just tick the letter. So it's a 1 in 4 chance as NewJudge says.
This would be different though if the answer was ¼ and the examiner allowed a and d as the correct answer.