Firstly the first part.
In magnetic fields we have learnt of three rules - Fleming's left hand rule, right hand rule and right hand grip rule.
The right hand grip rule is used to find the direction of the magnetic field (whether clockwise or anticlockwise) when a current flows through a conductor or vice versa. It can be also uses to find the direction of the magnetic field around a solenoid when current flows through it and vice versa.
Fleming's left hand rule is for the MOTOR EFFECT. Here the thumb is in the direction of the force(F), the index/first finger in the direction of the magnetic field(B) and the middle finger in the direction of current. (I). This can be easily learnt as FBI. Remember in the motor effect a force is produced.
The right hand rule is for ELECTROMAGNETIC INDUCTION. Here it is not a force that is produced, by an induced current. The first/index finger is the same as for the left hand rule but the thumb is now in the direction of motion. Remember not to mix these two up.
So in this question they have asked something to do with direction. So what of the three would you use? Here it is clearly electromagnetic induction as a current is induced - so we use the right hand rule.
If you point your thumb in the direction of motion (out of the page), your first finger in the direction of the magnetic field (downwards) then you see that the induced current flows from left to right. So that means electrons (electron flow is always the opposite of the direction of the current) flows from right to left. As a result the right side becomes positive!
Hope you understand it now
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