What is missing? If ([ABC]="123","321","1234"

The correct completion of the expression involves ensuring that all parentheses are properly opened and closed. In this case, the parentheses are used to define the boundaries of the function being executed. The function appears to be an If statement, which typically follows the syntax of If(condition, result_if_true, result_if_false).

In the provided expression, the opening parenthesis is present but there is no corresponding closing parenthesis. This leads to a syntax error if not rectified. The closing parenthesis is essential for the function to be interpreted correctly, as it marks the end of the logical condition and its respective outcomes.

While the other options might seem plausible in isolation, they do not address the syntactical requirements of the If function as effectively as the closing parenthesis does. The quotation marks and comma do not directly solve the structural integrity of the expression. As such, adding the closing parenthesis ensures that the If statement is complete and executable in the Quickbase environment.