Which of the following is a valid format for cell references in spreadsheets?

A valid format for cell references in spreadsheets typically consists of a column letter followed by a row number. In this case, the option "A1" perfectly fits that criterion, as it refers to the cell located in column A and row 1. This is the standard way to refer to cells in most spreadsheet applications, allowing users to easily identify and navigate to specific cells.

The other options do not conform to the conventional cell reference format. For instance, "6B" is not valid because it does not start with a letter, and traditional spreadsheet formats require the column to be indicated by a letter first. "34C" also fails because, similarly, it begins with a number instead of a letter. "R2C2" is an alternative referencing style known as R1C1 notation, which is valid in some contexts but not typically used as the standard cell reference format like A1 is. Therefore, "A1" is the most universally recognized and valid format among the given choices.