What is the most self-descriptive way to define a function that calculates sales tax on a purchase?

1.  def tax(my_float): '''Calculates the sales tax of a purchase. Takes in a float representing the subtotal as an argument and returns a float representing the sales tax.''' pass
2.  def tx(amt): '''Gets the tax on an amount.'''
3.  def sales_tax(amount): '''Calculates the sales tax of a purchase. Takes in a float representing the subtotal as an argument and returns a float representing the sales tax.'''
4.  def calculate_sales_tax(subtotal): pass

Correct Answer:

•  def calculate_sales_tax(subtotal): pass