This is the reason … i didn’t call you a) why b) since c) when

The correct option to fill in the blank is:

b) since

So the complete sentence would be: "This is the reason since I didn’t call you."

However, it's worth noting that "since" in this context may not be the most natural phrasing, and it could be clearer if rephrased slightly. A more common expression would be "This is the reason why I didn’t call you."