Please vote, but choose carefully. I will explain after some votes come in.
I would maybe rephrase the multiple choices or the question so as to better encompass the opinions people have on this matter. My opinion, for instance, is disparate from all 3 options you give. Not only that, but you should be more specific: “Does A God exist?” is different from “Does a SPECIFIC God exist?”
1) Well, I certainly do not believe God exists, so this certainly could not be my answer.
2) Well, I do have a belief that there is no God simply for practical reasons. I see no evidence for God and believe that if he were real, there would be evidence. However, when talking about reality, I cannot know for sure that there is no God because you cannot prove a negative (with few exceptions).
3) I very clearly do have an opinion on the matter but it does not fall within your two assigned options.
I am therefore unable to fill out the multiple choice question in an accurate way.
An even simpler question is “Do you believe god exists?”
This is a true yes/no question, unlike the above. This is the question I use to separate a theist from an atheist. If the answer is “yes” that is theism. If the answer is anything else (“no”, “maybe”, “I don’t know”) then that is atheism.
I answered no, inferring from the third option which mentions belief that this is a question about belief, and not about knowledge, in which case it becomes analogous to the true yes/no question “Do you believe god exists?”
I don’t believe god exists because I have no evidence that god exists.
The other question you can ask is “do you know god exists?” Questions of knowledge are more difficult. Consider the two questions: “do you believe your father is your father?” and “do you know your father is your father?”
I can easily say “yes” to the first one - I’ve grown up with my father since I was a little boy, etc.
I cannot easily easy “yes” to the second one. That kind of answer would need a DNA test, for example.
(In fact, my own father couldn’t answer “yes” to such a question, when he discovered in his 30s that his father - my grandfather - was not actually his father, but a substitute in order to avoid persecution. My grandfather had to hide his identity during WW2 because, even though he was not Jewish, he was mistaken for being a Jew, and therefore his son, my father, would have been persecuted as well.)
By “Is there a God?” you would be asking, “Do Gods or does God exist?” If anyone legally changed their name to God or someone named their kid God, then yes, God would exist. What do you mean by God?