Answer:
D). Linear with a positive slope.
Explanation:
Let x be the number of months.
We have been given that Howard puts $100 each month in a safe in his house to save for his daughter's college education. So the amount saved by Howard in x months will be 100x.
Since the initial amount is 0, so we can represent this information in slope-intercept form of equation we will get,

, where, y represents the total amount of money saved after x months.
As Howard saves $100 each month, so slope of our line will be 100. Therefore, the graph of Howard's investment will be a linear function with a positive slope (100) and option D is the correct choice.