Trump is set to replace which IRS chief commissioner after only two months?

The correct choice indicates that Trump was set to replace the IRS chief commissioner who had a brief tenure of two months. In the context of the choices provided, the accurate figure associated with a brief term as IRS chief commissioner is Charles Rettig. Rettig became the IRS commissioner in July 2018 and served through the Trump administration, completing his term well after two months.

The other names listed, such as Billy Long, Robert D. Marx, and David Kautter, do not match the timeline or the context regarding the IRS commissioner role during Trump's presidency. Kautter did serve as IRS acting commissioner before Rettig, but his role did not align with the brief two-month term. Therefore, the individual associated with a significant, short-lived appointment as I.R.S. commissioner would be Charles Rettig, making it the correct answer.