What time interval gives the half-life of X?

Answer: Option B

We usually encounter decay graph of the unstable radiaoactive substance. But this graph is the mass-time graph of the stable isotope. So the mass increases as time goes by.

What happens to the reading on the balance when current flows through the wire?

Answer: Option D

We always use FLHR to find the force on the wire. But magnetic forces is a action-reaction pair. So when the force acting on the wire is upwards, there is also an equal and opposite force acting downward on the magnet.

What is the change of potential difference between X and Y when the LDR is moved from dark to light room?

Answer: Option B

LDR (light dependent resistor) is a type of input transducer. When light intensity decreases, the resistance of LDR increases and vice versa.

You can redraw the circuit so that it looks like the usual potential divider that you are familiar with. Refer to the video below.