To calculate the cost of the app for \( m \) months, we need to consider both the joining fee and the monthly subscription cost.
- The joining fee is a one-time cost of $2.
- The monthly subscription cost is $5, and since it is for \( m \) months, we multiply this cost by \( m \).
The total cost \( c \) can thus be expressed as:
\[ c = 2 + 5m \]
So the correct expression that represents the cost of the app for \( m \) months is:
D) 2 + 5m.