The correct option is B about
The speaker explains that he's just going to be stepping out, anytime soon and asks if he can call back later. From the options given, 'about' is the most appropriate option, because the structure of the sentence, the placement of words comply with 'about', it means 'nearly, almost'. Hence option B is correct. It means 'almost', but the word 'almost' cannot directly replace option B. When we replace the word, the sentence sounds grammatically incorrect, and requires to be changed to 'I'm almost stepping out' or 'I'm almost going to step out', but we don't have the option of changing the sentence, hence option C is incorrect. 'Likely' refers to a certainty, it means 'probably', it makes the speaker sounds uncertain if he wants to step out or not, but the sentence does not express uncertainty, or else he wouldn't ask to call back later, had he been sure about being busy in the first place. Hence option A is incorrect. 'Soon' means 'immediately after' or 'quickly', and refers to time, whereas the sentence does not refer to time, it refers to degree, or an extent of an action 'about to step out'. If 'soon' is placed in the sentence, its structure would have to be changed 'I'm going to step out soon', but we don't have that option, hence option D is incorrect.