The correct answer to this question is "A.voy a una jineteada." When implying that you are going somewhere you would say "voy" as is "I am going". The other choices are incorrect because for B "Vas..." already implies you are saying that someone else will be going to the rodeo. C "Va" is saying that someone else is in the action of going to the rodeo at that time and D "van" is saying that multiple people will be going somewhere.
I hoped this answered your question!