Read this line from Frankenstein by Mary Shelley:

Her victory was announced by an unusual tranquility and gladness of soul which followed the relinquishing of my ancient and latterly tormenting studies.

Which definition of tranquility is most likely suited for this line?

Uncommon or unusual: Oxford English Dictionary

Anticipation or eagerness: Free Dictionary

Expectation: Merriam-Webster Dictionary

Calmness: peacefulness: Free Dictionary