The quantity of heat which crosses per unit area of a metal plate during condition depends upon
A. The density of the metal
B. The temperature gradient perpendicular to the area
C. The temperature to which the metal is heated
D. The area of the metal plate

Hint: To tackle this problem, we must first understand the rate of heat flow. The rate of heat flow is the quantity of heat transmitted per unit of time. We will discover the solution here by applying the heat flow formula.

Formula Used:
To find the heat flow the formula is,
\[Q = KA\dfrac{{\Delta T}}{L}\]
Where, A is a cross-sectional area, \[\Delta T\] is the temperature difference between two ends of a metal, L is the length of the metal plate and K is the thermal conductivity.

Complete step by step solution:
The quantity of heat is nothing but how much the heat flow per unit of time in a given material and is given by,
\[Q = KA\dfrac{{\Delta T}}{L}\]
As we can see here, the heat flow depends on the temperature gradient that is perpendicular to the area, that is it always depends on the difference in the temperature between the two ends of the metal and does not depend on the temperature to which the metal is heated. Therefore, the quantity of heat which crosses per unit area of a metal plate depends on the temperature gradient perpendicular to the area.

Hence, option B is the correct answer.

Note:Temperature gradients occur in solids, fluids, and as well as gases. For example, if the ends of an aluminium bar are exposed to two different temperatures, then, there exists a temperature gradient in the bar that causes the heat to flow from the hotter end to the cooler end.