The answer is C, but I don't understand how or why it is :/

Which part specifically do you not understand?

 

The degrees of freedom is the number of possible outcomes or classes (then subtract one). In this case, your question is referring to a dihybrid cross which normally has a 9:3:3:1 ratio which would means a dof of 3 (4-1=3).

 

Next you would follow the degrees of freedom to the highest possible number before you exceed 10, since that is your determined value. You could reject it at .01 but that would be leave you with another better choice that also, incidentally, increases your "assurance" that you did not mistakenly reject or accept your null hypothesis.

 

There are more than one answer to this question. You just have to look at all the options and choose the best one.

 

Hope that helps...