The distance of a galaxy is of the order of 1025 m. Calculate the order of magnitude of time taken by light to reach us from the galaxy.

The correct option is 3 × 10¹⁶ s.

To find the order of magnitude of the time taken by light to reach us from the galaxy, we can use the fundamental relation between distance, speed, and time:
$$\mathrm{Time}=\frac{\mathrm{Distance}}{\mathrm{Speed}}$$

Let's write down the given values:
1. The distance to the galaxy ($d$) is of the order of ${10}^{25}m$.
2. The speed of light in a vacuum ($c$) is approximately $3\times {10}^{8}m/s$.

Now, we substitute these values into our time formula:
$$t=\frac{{10}^{25}}{3\times {10}^8}$$

We can rewrite the expression to simplify the division of the powers of 10:
$$t=\frac{1}{3}\times {10}^{25-8}$$
$$t=0.33\times {10}^{17}s$$

To write this in standard scientific notation, we shift the decimal point one place to the right, which decreases the exponent by 1:
$$t\approx 3.3\times {10}^{16}s$$

This shows that the time taken by light to reach us is of the order of magnitude of 3 × 10¹⁶ s.