When an object is thrown up, the force due to gravity
A. acts in the direction of motion
B. acts opposite to the direction of the motion
C. remain constant as the body moves up
D. increases as the body moves up

Hint: In order to know the direction of force of gravity on an object when it's thrown upwards first we will discuss Acceleration due to gravity and force due to gravity on all objects on earth and then will determine the direction of force due to gravity. Also we should know that Force is the agent which is the cause of motion of any object in the universe, Force is the tendency to change the state of motion of objects.

Complete answer:
Since, we know that earth has a gravitational field and it causes a constant acceleration due to this field known as acceleration due to gravity on the surface of earth has its maximum value of $g = 9.8\,m{s^{ - 2}}$ . And this acceleration has a fixed direction, the direction towards the ground which is downward. It doesn’t matter which type of motion a body has, as long as it's present in the earth’s gravitational field the direction of acceleration due to gravity is always downward and a constant value on the surface of earth.

When an object is thrown up, its motion take place in upwards direction but the force due to gravity will always act in downward direction and force due to gravity is calculated as
$F = mg$ where m is mass of the body and g constant acceleration due to gravity.So, the motion of an object is in the upward direction, but the direction of force due to gravity is downward which is direction opposite to that of motion of object.

Hence, the correct option is B.

Note: It should be remembered that, the value of acceleration due to gravity remains constant on the surface of the earth but if we calculate the acceleration due to gravity to some height from the surface of earth its value decreases and $g = 0$ at the centre of earth. But remember the direction of acceleration due to gravity is always in downward direction towards the ground.