LEFT(Account.Zip_Code_c, 5) is the correct formula to display only the first five digits of Zip Code from the parent Account. LEFT function returns the specified number of characters from the left side of a text string. Account.Zip_Code_c is the custom text field that stores the Zip Code on Account object. 5 is the number of characters to return from the left side of the Zip Code. The other options are not valid formulas or functions.