What is defined as an opening in the Earth's crust through which molten lava, ash, and gases are ejected?

A volcano is a geological feature defined as an opening in the Earth's crust that allows for the eruption of molten lava, ash, and gases from beneath the Earth's surface. This process occurs when there is a buildup of pressure from magma in the Earth's mantle, which can eventually force its way through fissures or openings in the crust. When the magma reaches the surface, it erupts, leading to volcanic activity characterized by the release of various materials.

The other options represent different geological phenomena. An earthquake pertains to the shaking of the ground caused by the movement of tectonic plates, typically along faults, which are fractures in the Earth's crust where blocks of rock have moved past each other. A fault itself is a specific type of geological structure where these movements occur but does not involve the material ejection associated with volcanism. A trench typically refers to a deep, elongated depression in the ocean floor, often associated with tectonic plate boundaries where one plate is subducted beneath another, and does not relate to the ejection of lava or gases. Thus, the definition of a volcano distinctly aligns it with the described processes of molten material eruption, making it the correct answer.