Select the word which means the same as the group of words given.
'Something which cannot be understood.'

Here are the meanings of all the given options:
Incomprehensible: Not able to be understood; not intelligible.
Infallible: Incapable of making mistakes or being wrong.
Illegible: Not clear enough to be read.
Inexplicable: Unable to be explained or accounted for.
 
Out of the given words, we can see that only the word incomprehensible fits the meaning of the given words.
Hence, option 1 is correct.

Source: Oxford Dictionary