Select the most appropriate option to fill in the blank.

The clown was so funny ______ everybody burst out laughing.

that

The most appropriate option to fill in the blank is that.

The sentence is about the clown who was so funny that everybody burst out laughing. The word "that" is a subordinating conjunction that is used to introduce a clause that explains or provides additional information about the main clause. In this context, the speaker is saying that the clown was so funny that everybody burst out laughing.

The other options are not as appropriate.

• Because is a subordinating conjunction that is used to introduce a clause that explains the reason for something. This is not the most appropriate option because it does not explain why everybody burst out laughing.
• But is a coordinating conjunction that is used to introduce a clause that contrasts with the main clause. This is not the most appropriate option because it does not contrast with the fact that everybody burst out laughing.
• When is a subordinating conjunction that is used to introduce a clause that indicates time. This is not the most appropriate option because it does not indicate when everybody burst out laughing.