Final answer:
The merging of two neutron stars will form a black hole if their combined mass exceeds roughly eight solar masses, as neutron stars cannot support more mass without collapsing further due to the strong force.
Step-by-step explanation:
The primary factor that determines whether the merging of two neutron stars in a binary will form a black hole is a) The mass of the neutron stars. When two neutron stars merge, if their combined mass is greater than approximately eight solar masses, they will undergo further collapse to form a black hole. This is because neutron stars cannot sustain their structure due to the resistance provided by the strong force beyond this mass limit. Observations from instruments like the Chandra X-ray Observatory have helped to measure masses of various objects in space and identify those that are too massive to be neutron stars, thus inferring their identity as black holes. These measurements confirm that physics near a black hole is marked by intense gravity and tidal effects that can tear apart companion stars and produce X-rays from the accreted matter.