The company's customer service ratings had declined steadily for three consecutive years. ______, after a new training program was…

The passage describes a sequence in time: years of decline, followed by a specific intervention, followed by a sharp turnaround. Then correctly signals this straightforward temporal progression from one state to the next. Likewise would suggest the second sentence matches the first, but a sharp rise is the opposite of a steady decline. In particular would frame the second sentence as a more specific instance of the first's claim, which doesn't fit a sentence describing a reversal. Namely introduces a specific example of something just named, which doesn't fit this transition between two different time periods.