Modal verbs determine the mood of the sentence, like obligation (you must do it), permission (may I?), possibility (It might have been worse), ability (He can see it). Semi-modals or quasi-modals are similar to modals, they're common auxiliary verbs that express modality, or mood, of a sentence. The given sentence expresses the mood 'necessity', and asks the question 'is it required/necessary?'. 'Must' is used to indicate compulsion or determination, it does not refer to necessity, hence option A is incorrect. 'Ought' indicates moral obligation when placed in this sentence, something sounds amiss in 'do you really ought to', hence it isn't the suitable option, and C isn't correct. 'Used to' refers to a habit or custom, 'I used to play football in the backyard when I was younger', it does not provide the desired mood to the sentence, hence option D is incorrect. 'Need to' means 'necessary' and expresses the relevant mood of the sentence, which also asks if the "big step" is necessary hence option B is correct.