Work done is said to be positive when a force causes displacementA in its own direction  B in the direction opposite to the applied force C in the direction at right angles to the direction of applied force  D none of the above

To determine the correct answer, let's break down the options one by one:

A) "Work done is said to be positive when a force causes displacement in its own direction." This statement is correct. When a force causes displacement in the same direction as the applied force, the work done is considered positive.

B) "Work done is said to be positive when a force causes displacement in the direction opposite to the applied force." This statement is incorrect. When the force and displacement are in opposite directions, the work done is considered negative.

C) "Work done is said to be positive when a force causes displacement in the direction at right angles to the direction of the applied force." This statement is also incorrect. When the force and displacement are perpendicular to each other, the work done is considered zero.

D) "None of the above." Since option A is correct, we can conclude that option D is not the correct answer.

Therefore, the correct answer is A) in the direction of its own force.