The alternative that fits best is " d.Simultaneously"
Thus, the complete sentence would read: "All at once, pandemonium broke loose. Simultaneously the marching band came around the corner, dogs began barking, and the horses pulling the mayor's carriage began to run!"
The explanation for this is that the word "Simultaneously" is the only one that fits the phrase, and maintains the coherence and cohesion of the text. The words "anyway", "because", and "follwing", when embedded in the phrase, change the meaning of what the author wanted to go through.
