施行

新しい法律の ___ が来月から始まる。

The correct answer is 「施行」(しこう), meaning "enforcement" or "implementation" (of laws, ordinances, treaties). This word specifically refers to the official start of a law's effect. The sentence states "新しい法律の ___ が来月から始まる" (the ___ of the new law will begin next month), which perfectly aligns with the formal process of a law becoming active. 「実行」(じっこう) means "execution" or "practice"; while related, 実行 is more general for carrying out tasks, not specific to laws coming into effect. 「発行」(はっこう) means "issuance" (of documents, publications); laws are not "issued" in the same way. 「進行」(しんこう) means "progress" or "advancement"; while a process is advancing, 「進行」 refers to the ongoing movement, not the official start of legal validity.

正解は「施行（しこう）」です。法律が「効力を発揮し始める」ことを指し、「新しい法律の効力開始」が来月から始まる、という意味に最も合います。「実行」は物事を実際に行うこと、「発行」は書類などを出すこと、「進行」は物事が進むことで、法律の効力開始とは異なります。