Answer:
With
Step-by-step explanation:
The phrase "beside oneself with joy" is used to describe a state of extreme happiness or excitement. When someone is "beside themselves" with joy, it means they are so overwhelmed with joy that they are almost feeling disconnected from themselves or unable to contain their emotion. The word "with" is used to show what the person is feeling extreme joy about, in this case, it is joy. So, when we say "beside himself with joy," it means he is overwhelmed or overcome with joy.