That the larger volume of chimney will enhance natural convection is due to (a) higher thermal conductivity (c) larger radiation surface area (b) increase in the buoyancy force (d) increase in the volume expansion coefficient

Out of the four options provided, the most accurate answer is

option b) increase in the buoyancy force

Explanation:

Natural convection is a process in which thermal expansion of fluid takes place naturally due to natural buoyancy resulting in motion of fluid when it is heated.

Differences in densities result in buoyancy and natural convection depends on buoyancy force. Also higher air temperatures are found at lower densities which is found at the outlets of the channels and the larger the channel size, the larger is the buoyancy force (as the density difference will be higher).