To express the total cost \( c \) of driving \( m \) miles with Taxi B, you can multiply the cost per mile by the number of miles. Since Taxi B charges $1.50 per mile, the expression for the total cost can be written as:
\[ c = 1.50m \]
So, the expression representing the cost of driving \( m \) miles is:
\[ c = 1.50m \]