NOTE:There is a difference between the value you gave for cost of text messages in the question and the values given in the group of answers. I will use $0.021 for cost of text messages since it is common to all answer choices.
Total cost = Cost of unlimited calls + cost of text messages
C = $40 + $0.021
= $40.021
Therefore, the equation which represent the total cost in dollars for a month is 40.021