The phrase "La ilaha illa Allah" translates to "There is no deity except Allah." This declaration is the first half of the Shahadah (the Islamic declaration of faith), which is the first pillar of Islam. It affirms the concept of Tawhid - the absolute oneness and uniqueness of Allah as the only deity worthy of worship.
Breaking down the phrase: "La" means "no" or "there is not," "ilaha" means "deity" or "god," "illa" means "except" or "but," and "Allah" is the proper name of God in Islam. Together, the phrase negates all false objects of worship and affirms that only Allah deserves worship.
The other options describe different concepts. "Muhammad is the Messenger" (Muhammadur Rasulullah) is the second half of the Shahadah, not the first. "There are many gods" directly contradicts the meaning of the phrase, which explicitly denies the existence of any deity besides Allah. "Allah is the greatest" (Allahu Akbar) is the Takbir, a separate declaration used in prayer and other contexts.