Answer: a. A fluid’s resistance to flow
Explanation:-
Viscosity is defined as the resistance to the flow of a fluid. More are the inter molecular forces between the particles of a liquid, the more is the viscosity of the liquid and thus it will flow slowly.
For example: Oil is more viscous than water and thus flows slowly than water. Thus it means the inter molecular forces among oil are much stronger as compared to the inter molecular forces among water.
Viscosity is measured in S.I. units of poise.