The sentence requires a subject and verb to complete the clause. 'It is an entirely different world' correctly provides a pronoun and verb that refers back to the military context.
Chemical elements generally do not take a definite or indefinite article when referred to in a general sense. 'Hydrogen' is the correct noun to use as the subject.
The subject of the main clause is Vincent van Gogh, so the pronoun in the dependent clause must match the gender and number of the subject. 'He' is the correct pronoun.
The sentence needs a subject and verb to complete the clause following 'investigation'. 'Proved that the space shuttle' provides the necessary structure.