That room's window is left open. [Syntax]

The correct answer is "窓(まど)が" (mado ga). This sentence uses the pattern "V-てあります" (something has been V-ed / is in a V-ed state), which indicates a resulting state from an intentional action. The action is "開(あ)けてあります" (has been opened). The subject of this state is "窓(まど)" (window), which is marked by the subject particle "が". Therefore, "あの部屋(へや)の窓(まど)が開(あ)けてあります" means "The window of that room is left open." "部屋(へや)" and "の" are already in the sentence, connecting "あの" and "窓". "開(あ)けて" is the te-form verb and comes after the subject.

正解は「窓が」です。「〜てある」は、ある行為の結果の状態が続いていることを表します。この場合、「開けてある」のは「窓」なので、「窓」が主語となり、助詞「が」で示します。「部屋」と「の」はすでに文中にあり、「開けて」は主語の後ろにくる動詞のため不適当です。