The first and the third. :) Don't worry, we only have four. However, we also mix them.

I didn't know the context was the Arctic. :)

Yes, you can use a second conditional here, and your placement of "not" is perfectly right. It makes me think that it isn't currently summer. A zero conditional states this as a fact.

Thanks for the explanation.

What other types of conditional sentences, except the second and zero?

Thanks, but I'm not sure. Probably it should be something like this:

If it was summer, it would not be dark outside. (talking about the Arctic)

You have a zero conditional here. (You can look it up. That will show you the forms of the verb.)

Is it usually dark during the summer? :)