Help the clumsy chef complete his apology note to the bride.

I was happily carrying the magnificent five-tier wedding cake out of the kitchen _____ I suddenly slipped on a stray meatball.

The correct answer is when.

We use when to introduce a short action (past simple) that interrupts a longer, continuous action in the past (past continuous). In this case, slipping on the meatball interrupted the continuous action of carrying the cake.

If we used while, it would need to be followed by a continuous action (e.g., "...while I was carrying the cake").