Peter F. answered 04/22/22
Multiple Elementary Subject Teacher; 18 Years' Teaching Experience
Hello Nathan,
First of all, it should be duly noted that sentences A and B are the same as one another, and sentences C and D are the same as one another. Therefore, we can eliminate sentences A and D right off the bat since they are duplicates of sentences B and C.
So between sentences B and C: the correct one is sentence C because "called when he said he was going to call" describes something that the subject's dad did in accordance with the previous two actions taken by the dad (grammatically speaking) that caused the subject's mom to fall in love with him.
Whereas "and she could could count on him calling when he said he was going to call" as written in sentence B shifts the focus from the dad back to the mom in terms of the mom becoming the subjective focus of the sentence instead of the dad, in contrast to the previous two actions written in the sentence ("he always told her she was important to him, spent all of his time after school with her,") having been performed by the dad instead of the mom accordingly.