Efficiency is completing a task as fast a possible the the least amount of effort.
As an airplane moves through the air, its wings cause changes in the
speed and pressure of the air moving past them. These changes result in
the upward force called lift.
The Bernoulli principle states that an increase in the speed of a fluid
occurs simultaneously with a decrease in the pressure exerted by the
fluid.
A wing is shaped and tilted so the air moving over it moves faster than
the air moving under it. As air speeds up, its pressure goes down. So
the faster-moving air above exerts less pressure on the wing than the
slower-moving air below. The result is an upward push on the wing—lift!
To solve this problem it is necessary to apply the concepts related to the Stefan-Boltzmann law which establishes that a black body emits thermal radiation with a total hemispheric emissive power (W / m²) proportional to the fourth power of its temperature.
Heat flow is obtained as follows:

Where,
F =View Factor
A = Cross sectional Area
Stefan-Boltzmann constant
T= Temperature
Our values are given as
D = 0.6m

The view factor between two coaxial parallel disks would be


Then the view factor between base to top surface of the cylinder becomes
. From the summation rule


Then the net rate of radiation heat transfer from the disks to the environment is calculated as





Therefore the rate heat radiation is 780.76W
Answer c, velocity would be the answer.