Choose the correct word/phrase to complete the sentence. I like travelling by air ________ the weather is bad.

Unless is the right word.
'While'is a conjunction. Its meaning varies depending on the context in which it is used. When used as a noun, its meaning can be expressed across time, as in "We chatted for a while." When it connects two phrases or sentences as a conjunction, it suggests a contrast, such as She wanted to eat rice while I was rooting for pasta.It is impossible to enjoy plane travel when the weather is poor.
If we look at choice 2, 'because' it will also not be the correct option'because' the weather is bad, one person cant like travelling by air.
The word 'when' refers to the time period. It is also not conceivable according to the same reasoning.
As a result, the word 'unless' is the only option that will be correct here. Unless is used in a negative manner, as if the weather is not bad, the person is narrating his enjoyment of air travelling.