Answer :
Cambridge English Dictionary defines "terrible" as "very unpleasant or serious or bad."
So, terrible is a synonym of feeling bad. In the given paragraph, the lines " Ian felt terrible." and "He felt very badly" are repetitions of each other.
The sentences " Ian felt terrible. He felt very badly about not watching his sister. He made a resolution to be a better brother in the future. " in this paragraph can be combined to eliminate repetition most effectively in the following way :
A) Ian felt very badly about not watching his sister, so he made a resolution to be a better brother in the future.