Step 3: Compare the given options to the correct designation of linoleic acid. The correct designation is '18:2 Δ⁹,¹²', which matches the structure of linoleic acid. The other options describe different fatty acids: '18:1 Δ⁹' refers to oleic acid, '20:4 Δ⁵,⁸,¹¹,¹⁴' refers to arachidonic acid, and '16:0 Δ⁹' is not a valid designation for a saturated fatty acid.