Judgment of the Court (First Chamber) of 18 March 2021.#MK v Autoridade Tributária e Aduaneira.#Request for a preliminary ruling from the Tribunal Arbitral Tributário (Centro de Arbitragem Administrativa - CAAD).#Reference for a preliminary ruling – Direct taxation – Tax on capital gains from immovable property – Free movement of capital – Basis for assessment of tax – Discrimination – Option to be taxed according to the same arrangements as residents – Compliance with EU law.#Case C-388/19.