Answer:
The best option to modify this sentence is: change then to than.
Step-by-step explanation:
There is no mistake in the way the sentence is punctuated. Also, "calm" is an adjective referring to lake, so it does not need to be changed. "Calmly" is an adverb, and would be used differently.
However, there is a misspelling in "then". The correct word is "than", which a part of the expression "would rather... than...", which expresses preference. That is the only problem with the sentence. The correct form is:
If I were a duck, I would rather swim in a calm lake than a fast moving stream.
Another example with "would rather... than..." would be:
She said she would much rather not buy it at all than borrow the money to do it.