Final answer:
The total manufacturing cost is calculated by adding the direct materials cost, direct labor cost, and variable manufacturing overhead cost.
Step-by-step explanation:
In the question, the direct materials cost is $5.40, direct labor cost is $2.90, and variable manufacturing overhead cost is $1.60. To calculate the total manufacturing cost, we need to add these three costs. Therefore, the correct option is d) Total manufacturing cost = $8.90.