The best answer to this question (of those given) is D, Congress.
When George H. W. Bush was running for president, he told people to "read my lips" and promised that there would be no new taxes imposed on them. However, the economy did not do as well as Bush's people had projected and there was not as much revenue available to spend on government programs. Bush then faced the choice of raising taxes or cutting spending on programs that were quite popular. The Democrats who controlled Congress would not agree to any major spending cuts unless some taxes were raised.
Because of this, it is most accurate to say that Congress forced Bush to break his promise.