Which of the following bonds is totally tax-exempt?

The correct choice is Puerto Rico GO bond, as it is completely tax-exempt at both the federal and state levels for U.S. taxpayers. This tax-exempt status is granted due to the unique financial and political situation of Puerto Rico, which allows its bonds to be exempt from federal taxation.

This characteristic is appealing to investors who are looking to minimize their taxable income, especially if they fall into higher tax brackets. Hawaii GO bonds and US government bonds do have certain tax advantages, like exemption from state and local taxes, but they are not entirely tax-exempt at the federal level. US Steel bonds are corporate bonds and are subject to federal, state, and local taxes, making them fully taxable. Therefore, the Puerto Rico GO bond stands out as the only option that offers complete tax exemption.