Explanation:

The correct answer is option 'b' - about.

The given sentence is:
"I'm just _____ to step out. Can I call you back later?"

In this sentence, the speaker is indicating that they are about to step out or leave. They are asking if they can call the person back at a later time.

Let's analyze the options:

a) likely: This option refers to something that is probable or expected to happen. However, in the given sentence, the speaker is not expressing likelihood but rather indicating their immediate intention to step out. Therefore, this option is not suitable.

b) about: This option means "to be on the point of doing something" or "to be ready or prepared to do something." It perfectly fits the context of the sentence as the speaker is expressing their intention to step out. Hence, this is the correct answer.

c) almost: This option means "nearly" or "very close to." While it is similar in meaning to 'about,' it does not convey the same sense of immediacy. The speaker in the sentence is not indicating that they are nearly ready to step out; instead, they are specifying that they are just about to do so. Therefore, this option is not the most appropriate choice.

d) soon: This option refers to an action or event happening in the near future. However, it does not convey the same sense of immediacy as the word 'about' in the given sentence. The speaker is indicating that they are just about to step out, rather than stating that they will do so in the near future. Hence, this option is not the most suitable.

In conclusion, the most appropriate word to fill in the blank is 'about' (option b) as it accurately conveys the sense of being on the point of stepping out.