Which of the following is a run-on sentence?

a.
I was surprised by how loud my new baby sister could cry I might have to get ear plugs.

b.
It was such a beautiful day, so we decided to eat lunch outside.

c.
My favorite things to do on rainy days are to eat popcorn and watch movies.

d.
The principal was concerned about the situation; he called a special meeting with the teachers.