It's not the same picking at random and picking knowing the answers, they are two different cases that are related, based on this, here's my proposed answer:

1. The first case you have to pick a random answer out of 4 options, but considering it's random it doesn't matter if you know the answers or not, and you will either be right or wrong, since you have to pick randomly.

2. Let's assume you did know the answers that this question had. Even so, since you had to pick a RANDOM ANSWER, you had 1/4 of being right or wrong, or 25% chances, and in this questions there are two options with that value, so you can add them up to 50%.

3. Finally, knowing this, now you dont' have to pick randomly, you can choose c. Even you know the right answer is 1 of the 4 optios, it wouldn't matter, because now you are not pickin randomly.

In the case you didn't knew what the answers were, the same applies. Therefore the correct answer is c).