The correct option is def test_apply_discount(): assert apply_discount(120, 0.25) == 90 assert apply_discount(240, 0.1) == 216 assert apply_discount(60, 0) == 60 assert apply_discount(0, 0.3) == 0 assert apply_discount(-80, 0.1) == -72 because it verifies normal percentage calculations and it explicitly covers zero and negative edge cases.

This set exercises typical inputs so it confirms that common discounts compute as expected with 120 at a quarter rate producing 90 and 240 at a tenth rate producing 216. It also checks a zero rate which should leave the amount unchanged and a zero amount which should remain zero regardless of the rate. Including a negative amount is important for scenarios like refunds and the arithmetic stays consistent since subtracting ten percent from negative eighty yields negative seventy two.

def test_apply_discount(): assert apply_discount("150", 0.1) == "135" assert apply_discount(150, "0.1") == 135 is incorrect because it passes strings for the amount or the rate and expects a string result which conflicts with the numeric behavior of the function and would raise a type error. It also fails to include zero or negative cases.

def test_apply_discount(): assert apply_discount(150, 1.5) == -75 assert apply_discount(150, 0.2) == 30 is not suitable because the second expectation is wrong since a twenty percent discount on one hundred fifty leaves one hundred twenty. It also skips zero and negative cases and the rate greater than one does not reflect the typical percentage inputs you want to validate here.

def test_apply_discount(): assert apply_discount(200, 0.1) == 180 assert apply_discount(80, 0.5) == 40 assert apply_discount(50, 0.2) == 40 is incomplete because it only checks normal scenarios. It omits the zero rate path and it does not test zero or negative amounts which the prompt specifically calls out as edge cases to include.

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