Explanation:

- The ratio of P to Q in the alloy is given as 3:7. This means for every 3 parts of P, there are 7 parts of Q.

- The total parts are 3 + 7 = 10.

- If the total alloy weighs 1800 kg, we need to determine how much of this is made up of Q.

- Q represents $\frac{7}{10}$ of the total weight.

- Calculate $\frac{7}{10}\times 1800=1260$ kg.

- Correct Answer: Option 4 - 1260 kg