To convert letter grades into a 4.0 scale based on user input, the most appropriate programming construct to use would be the if-then-else statement. This construct allows you to evaluate the input letter grade and execute different code blocks depending on the grade entered.
For example, you could use it like this:
grade = input("Enter your letter grade: ")
if grade == "A":
gpa = 4.0
elif grade == "A-":
gpa = 3.7
elif grade == "B+":
gpa = 3.3
# Add other conditions as necessary
else:
gpa = 0.0 # or some error handling
print(f"Your GPA is: {gpa}")
So, the answer is if-then-else.