The longest term among the options provided is "cadence." In terms of letter count, "cadence," "climax," and "motive" all have the same number of letters (7), while "phrase" has 6. However, if you're considering the context of length in terms of musical or narrative elements, you may be looking for something else.
If the question is intended to determine the length by letter count, then there's no single answer as several terms have the same length. If it's by a different context, please clarify!