Step 1: What the question is about.
When you rub polythene with wool, tiny particles called electrons jump from one body to the other. Polythene gets a negative charge, so it has gained extra electrons. We need to count how many electrons made this jump.
Step 2: The rule about charge.
Charge is not smooth. It comes in tiny equal packets. Every packet is one electron, and one electron carries a charge $e = 1.6 \times 10^{-19}$ C. So the total charge is just the number of electrons times this small packet.
Step 3: Write the simple formula.
If $N$ electrons move, the total charge is
\[ q = N \times e \]
We want $N$, so we turn the formula around:
\[ N = \frac{q}{e} \]
Step 4: Put in the numbers.
Here $q = 4 \times 10^{-7}$ C and $e = 1.6 \times 10^{-19}$ C. So
\[ N = \frac{4 \times 10^{-7}}{1.6 \times 10^{-19}} \]
Step 5: Do the easy division.
First divide the front numbers: $4 \div 1.6 = 2.5$. Then handle the powers of ten by subtracting them: $-7 - (-19) = 12$. So
\[ N = 2.5 \times 10^{12} \]
Step 6: State the result.
About two and a half trillion electrons moved from the wool onto the polythene. This is option (4).
\[ \boxed{N = 2.5 \times 10^{12}} \]