Explanation:

- Option 1: "two year's" – This is incorrect because it uses the possessive form. We need an adjective form without the apostrophe.

- Option 2: "two years" – This is incorrect because "years" in this form implies a plural noun, not the adjective form needed with "course."

- Option 3: "two year" – This is correct. It uses the adjective form to describe the course length, similar to "two-week vacation."

- Option 4: "one years" – This is incorrect because it mixes singular and plural. "One" should be followed by the singular "year."

Given these explanations, Option 3: "two year" is indeed the correct choice.