When cleared for the ILS RWY 18 at Lincoln Municipal and crossing the Lincoln VORTAC at 5,000 feet MSL, when can descent to 3,200 feet commence?

When cleared for the ILS RWY 18 and crossing the Lincoln VORTAC at 5,000 feet MSL, descent to 3,200 feet can commence immediately following the clearance. This is because the initial descent is allowed once the aircraft is on the appropriate segment of the approach and within the confines of the airspace.

At this point, the pilot has the necessary clearance and is permitted to begin descending to the next altitude as per the approach procedure. The approach procedure is designed to ensure safe and controlled descent rates, and starting the descent immediately upon clearance optimally aligns the aircraft with the subsequent phases of the approach.

Other options reflect different points in the approach where descent might logically occur, but they do not correspond to the instruction provided when cleared for the ILS approach. For example, waiting until reaching the decision altitude does not align with standard procedure as it implies maintaining the higher altitude unnecessarily. Moreover, considering the final approach fix or the VORTAC as points to begin descent can misalign with the intended progressive reduction in altitude that is standard practice when following instrument approach procedures.