The question asks which type of function returns a single value per invocation. According to Snowflake documentation and the community discussion, a scalar function returns exactly one value per row/invocation. This contrasts with aggregate functions (which return one value per group of rows), table functions (which return multiple rows), and window functions (which operate over a window of rows but return one value per row, though they are a specific category). The community discussion with 100% consensus and upvotes confirms that scalar functions are the correct answer, as they are designed to return one value per invocation regardless of the input row count.