Differential equations are mathematical equations that involve derivatives of an unknown function. They are widely used in various fields of science and engineering to model and analyze dynamic systems. In this question, we are asked to identify the correct option regarding the nature of the functions involved in differential equations.

Differential equations involve functions y = f(x) and g(x), where y represents the dependent variable and x represents the independent variable. These functions can have various properties and behaviors based on the given equation. Let's analyze each option to understand why option 'C' is the correct answer.

a) Tangent of y at zero:
The tangent of a function at a specific point represents the slope of the function at that point. This option suggests that the differential equation involves the tangent of y at zero. However, this is not a general characteristic of differential equations. Differential equations can involve tangents at any point, not just zero.

b) Maxima of y:
Maxima of a function refers to the points where the function reaches its highest values. While differential equations can involve maxima or minima, it is not a general property of all differential equations. Therefore, this option is not correct.

c) Derivatives of y:
This option suggests that the differential equations involve derivatives of y. This is the correct answer. Differential equations often involve derivatives of the unknown function y with respect to the independent variable x. By including derivatives, we can describe how the rate of change of y varies with respect to x.

d) Minima of y:
Minima of a function refers to the points where the function reaches its lowest values. Similar to option 'b', this is not a general property of all differential equations. While some differential equations may involve minima, it is not a defining characteristic.

In conclusion, the correct answer is option 'c' because differential equations commonly involve derivatives of the unknown function y. Including derivatives allows us to describe the rate of change and behavior of the function with respect to the independent variable.