Which string function returns a Boolean value as a result?

STARTSWITH is the function that returns a Boolean result, so D is correct. The function evaluates whether one string begins with a specified substring. Its conceptual syntax is STARTSWITH(string, substring). If the original string begins with that substring, Tableau evaluates the expression as TRUE; otherwise, it evaluates as FALSE. For example, STARTSWITH([Customer Name], "A") returns TRUE for values beginning with A and FALSE for other values. Because the output is Boolean, STARTSWITH is particularly useful in calculated fields, conditional expressions, filters, classifications, and data-quality logic. The other listed functions return different data types. CHAR converts a numeric character code to its corresponding character and therefore returns a string. FIND searches for the position of a substring and returns a numeric position. UPPER converts the supplied text to uppercase and returns another string. Consequently, only STARTSWITH directly produces the TRUE/FALSE result required by the question. This distinction is fundamental when selecting functions for Boolean calculations because Tableau evaluates Boolean fields differently from numeric and string-returning expressions.