Question

how do deep ocean trenches form?
when continental crusts collide
when oceanic crusts collide
when continental crusts subduct under less dense oceanic crusts
when oceanic crusts subduct under less dense continental crusts

Explanation:

Deep - ocean trenches form at subduction zones. Oceanic crust, which is denser than continental crust, sinks or subducts beneath the less - dense continental crust. This process creates deep depressions in the ocean floor known as trenches.

Answer:

When oceanic crusts subduct under less dense continental crusts