Explanation:

- Let's call the distance between home and office "D" km.

- Let the normal time taken to reach the office be T hours.

- At 3 km/hr, he is 40 minutes (2/3 hour) early: so time taken = T - 2/3 hours.

- At 2 km/hr, he is 40 minutes (2/3 hour) late: so time taken = T + 2/3 hours.

So,

D/3 = T - 2/3

D/2 = T + 2/3

Now, subtract the first equation from the second:

D/2 - D/3 = (T + 2/3) - (T - 2/3) = 4/3

D/2 - D/3 = D/6

So, D/6 = 4/3

Therefore, D = 8 km

Option 2: 8 km is correct.