What does a PAPI primarily assist with during landing?

A PAPI, or Precision Approach Path Indicator, primarily assists pilots by providing a visual indication of the aircraft's position in relation to the ideal glide slope during landing. It consists of a series of lights located beside the runway that indicate whether the aircraft is too high, too low, or on the correct approach path. This system enhances safety by guiding pilots to maintain the proper descent angle, which is critical for a successful landing.

The other options do not relate to the primary function of PAPI. For instance, fuel efficiency calculations, runway maintenance tracking, and flight plan submissions are aspects of aviation operations but do not directly involve the assistance provided by PAPI during the landing phase. The focus of PAPI is specifically on visual guidance for landing aircraft, making option B the correct choice.