Option(a):

In an ac circuit power is given by:

where V_rms is the rms voltage and I_rms is the rms current and

Now, ϕ is constant for a particular ac connection. If Power is constant, rms current and rms voltage have an inverse relation. Thus, higher voltage means lower current. Thus, this option is correct.

Option(b):

We have established that we get a lower current. Now, the power dissipated (not the net power supplied by the source) through the transmission wires of resistance R is given by:

As I is lower, P_dissipated is also less. Hence, this option is correct.

Option(c):

Making the transmission lines thinner is not a good idea. As resistance is inversely proportional to area, the resistance will increase. Hence energy dissipated will also increase( ). Thus, this is incorrect.

Option (d):

This is true. We can use transformers to increase or decrease voltages and decrease or increase current respectively. This is correct.