The value that will appear in the crosstab for the Consumer segment of the year 2018 if you change compute to use Order Date is 0. The Last() calculation is a table calculation that returns the number of rows from the current row to the last row in the partition. The partition is determined by how you compute the calculation. In this case, you change compute to use Order Date, which means that the calculation will be performed for each value of Order Date.
The crosstab shows that there are two values of Order Date for the Consumer segment of the year 2018: Q1 and Q4. The Last() calculation will return 0 for Q4, because it is the last row in the partition. It will return 1 for Q1, because it is one row away from the last row in the partition.
The other options are not correct for this scenario. The Last() calculation will not return 2, 3, or 4 for any value of Order Date, because there are only two values in each partition.