In the following code:

public class car $\{$

private string carName;

private double carprice;

private string carcolor;

private Engine engine;

$\}$

a$.$ Engine is an object of class Car

b$.$ The compiler will report an error because of declaring the engine in the Car class

c$.$ engine will be defined per the Car class $($as a class variable, not per object$)$

d$.$ engine will be an instance field of each Car object

clear my choice