The correct option is A I was just about to go
The context refers to an action that took place in the past and had already been completed.
Thus, this is in simple past tense. The simple past tense uses the past participle of the verb.
Here, "I was just about to go" concurs with the context as it implies an action that has already been completed in the past. Thus, option A is the correct answer.
"I would just go" implies a wish or preference. This is incorrect in the context.
"I'm just about to go" is in simple present tense and does not concur with the context.
"I've just about gone" is in present perfect tense and is incorrect in the given context.
Thus, option A is the correct answer.