The correct answer is B. This is calculated by firstly adjusting the overhead cost from 2 years ago to current price levels by multiplying by 155/121, to obtain a cost of $4,740.
This figure is then used in a high low calculation (change in cost divided by change in activity) to obtain the variable cost per unit (($13,000 - $4,740) / (3,000 units - 1,000 units) = $4.13).
The most popular choice was alternative C, which was selected by majority of candidates. This indicates that although competent in the high low technique they failed to adjust costs to current price levels. In analysing cost data it is important that inflation is allowed for. Those who chose option D indicated that either they guessed badly, or that they could competently perform the high low calculation and that they realised a need to adjust the figures for inflation but failed to do so correctly and multiplied by 121/155). Finally a minority chose alternative A, again possibly suggesting a bad guess or alternatively that they indexed costs to price levels from two years ago.