In natural convection heat transfer, the parameter
a) nusselt number
b) rayleigh number
c) grashof number
c) lewis number

correct answer is option c i.e Grashof Number

Explanation:

The Grashof number is a dimensionless number, which is named after renowned scientist  Franz Grashof. The Grashof quantity is defined as the proportion of the buoyant force to viscous force performing on a fluid in a pace boundary layer.

Its function in natural convection is more or less the same as that of Reynolds's number in compelled convection.