Concept:This sentence requires the past perfect tense to show that an action was not completed before another past action.
Explanation:The clause "when I entered college" is in the simple past tense.
So the action concerning the couple's first child must refer to a time before that past moment.
The past perfect form is constructed as "had + past participle".
For the verb "have", the past participle is "had".
Thus, the correct negative form is "had not had", contracted as "hadn't had".
This clearly means that, at the time the speaker entered college, the couple still did not have their first child.
Option A "haven't had" and Option C "hasn't had" are present perfect, which do not match the past time frame.
Option D "were not having" is past continuous, which does not express the completed-before meaning needed here.
Answer:The correct answer is B: "hadn't had".