If you just want to know yhe exact translation of " It is very cold", then the answer is either 5 or 6. We usually say 5 or 6. The others are odd and unnatual.

1. 这是冷的： when i have salad for breakfast, i would say "这是冷的", emphasize the thing i touch, or i eat, or i feel is cold, 2. 这很冷： same usage as "这是冷的"， but it means colder 3. 这非常冷 : same usage as "这是冷的" , but it means coldest, sounds that you are upset, not satisfied with something. 4. 天气是冷的: talk about the weather. the weather is cold. a little odd. 5. 天气很冷: same usage as "天气是冷的"， but it means colder. sounds natual, you think the weather is cold. 6. 天气非常冷：same usage as "天气是冷的" , but it means coldest，emphasize the weather is extremely cold. sounds you need sweater.

你好，其实你提的问题很好，很有典型的意义。 这么说吧，汉语的语义是包罗万象的，用法很活。 以上你说的在不同的语境都是可以的，单独说一个句子脱离语境都不能说对错，这个和英语不同，所以学习汉语思路应当是活的，而不是很教条，希望对你有帮助。

天气很冷or 天气非常冷are natural。 这is a demonstrative pronoun, if listeners and speakers are aware what they talk about,the speakers dont have to mention it, for example, they know they talk about the fish, the speaker can simply say 这不好吃, and the listener would know that "the fish isnt tasty.", so it is ok to say 这很冷/非常冷 in some cases.

天气非常冷and天气很冷are more resonable.And as a Chinese i use these two words very frequently .If you said 天气是冷的，It's right but sounds a bit weird .