The sentence that correctly follows comma conventions is: "After he finishes his breakfast, he will go for a walk around the block."
The other sentences have comma issues:
- "We will go to the museum and then we will go out to dinner." could use a comma before "and" for clarity, but it is a matter of style.
- "I prefer chocolate ice cream over vanilla" said the boy. should have a comma before "said" to separate the dialogue from the attribution.
- "My sister Marcie, who loves to read is spending the day at the library." is missing a comma before "is" to properly set off the non-restrictive clause.