The definition of tranquility that is most likely suited for this line is calmness: peacefulness.
That is what the word tranquil means.
After he is helped back to his stall, Squealer informs them that Napoleon has sent for the veterinarian at Willingdon to treat him. When the van arrives to take Boxer to the hospital, however, Benjamin reads its side and learns that Boxer is actually being taken to a knacker, or glue-boiler.
It's either A or C. Complex sentences doesn't fit in with it.
It means the best of the best. Cannot get any better. Amazing, Wonderful!