The expense for the year ended 31 December 2018 would be calculated based on the number of expected to vesting options, which is adjusted for the revised estimate of employee attrition. Initially, Tucan expected 2 out of 10 employees to leave before the end of the service period, which means they expected 8 out of 10 employees to vest (80%). Since no one left by the end of 2018, the expected number of vesting options remains at 80% (8 out of 10). The expense for the year would be calculated as follows: (800 options * 20, the fair value per option) * (1/5), the proportion of the vesting period that has passed by the end of 2018.

So, the expense for the year ended 31 December 2018 would be:

( 800 \times 20 \times \frac{1}{5} = 3200 \times 1 = 3200 )

However, since the expense is recognized over the vesting period, we need to adjust for the time elapsed in 2018, which is 1/5 of the total vesting period (5 years). Therefore, the expense for 2018 would be 1/5 of the total cost of 3200, which equals 640.

Thus, the correct answer for the expense for the year ended 31 December 2018 is:

( 640 )

This amount should be recognized as an expense in the income statement and a corresponding credit recorded in a liability account, such as "Share-based compensation reserve" in equity.