Choose the incorrect statement
(A) Fleming's right –hand rule is a simple rule to know the direction of induced current
(B) The right – hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors
(C) The difference between the direct and alternating currents is the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.
(D) In India, the AC changes direction after every $\dfrac{1}{{50}}$ second

Hint: Fleming’s right-hand rule states that when a conductor moves in a magnetic field the direction of induced current can be shown using our right hand. The thumb, the first finger, and the middle finger are mutually perpendicular to each other. The thumb shows the direction of the conductor’s motions, the first finger shows the direction of the magnetic field, and the middle finger shows the direction of the induced current.
The dc does not change its direction whereas the ac always changes its direction like a wave.
Note that the electricity moves in one way direction for a little moment and then reverses during the different position of the generator armature.

Complete step by step answer:
We are given some statements in the question from which we have to find the incorrect one. So, we should go through all the given options and analyze them.

> $Option (A)$ Fleming's right –hand rule is a simple rule to know the direction of an induced current-
This statement is true because we can see the direction of the induced current which is occurred due to the movement of a conductor in a magnetic field, by using Fleming’s Right-hand rule. This rule states that if we keep the thumb, the first finger, and the middle finger of our right hand mutually perpendicular to each other, the middle finger shows the direction of the induced current.

> $Option (B)$ The right – hand rule is used to find the direction of magnetic fields due to current -carrying conductors-
This statement is true because if we keep the thumb, the first finger, and the middle finger of our right hand mutually perpendicular to each other, according to Fleming’s right-hand rule the first finger shows the direction of the magnetic field due to the current-carrying conductor.

> $Option (C)$ The difference between the direct and alternating currents is the direct current always flows in one direction, whereas the alternating current reverses its direction periodically-
This statement is true because, if we send dc electricity to a circuit it does not change with time and always remains in one direction. Its direction does not reverse. But for ac electricity, it changes its direction with a certain time-period like a wave.

> $Option (D)$ In India, the AC changes direction after every $\dfrac{1}{{50}}$ second-
The electricity moves in one way direction for a little moment and then reverses during the different position of the generator armature. In India, the standard household current is in $50Hz$. The direction changes two times in one cycle, hence the direction change becomes after every $\dfrac{1}{{100}}$ second. So this statement is wrong.

Hence, the correct answer is option (D).

Note: Fleming’s right-hand rule- If we take the first finger, middle finger, and the thumb are kept mutually perpendicular to each other, then the first finger represents the direction of the magnetic field and the thumb shows the direction of motion of the conductor, the middle finger denotes the direction of induced current. This rule is also called the Dynamo Rule.