To put the sentence in formal style, which is the best revision to make? changing “undertake” to “go for” changing “it’s” to “it is” changing “investigate” to “check out” changing “job” to “gig”

In order to put the senten in formal style, the best revision to make is changing “it’s” to “it is.” So, option B is correct.

Contractions in texts, turn it into an informal style writing.

Formal language is less personal than informal language. It is used when writing for professional or academic purposes like university assignments.  Formal language does not use colloquialisms, contractions or first person  pronouns such as ‘I’ or ‘We.’ On the other hand, Informal language is more casual and spontaneous.

marriage difficulties adversely affect parent-child communication

- the heat nearly killed them that first summer.