The diagram below shows the basic materials and processes involved in the rock cycle.

Based on the provided options relating to the rock cycle, the true statement is:

**D. All three types of rock can be weathered to form sediment.**

Here's a brief explanation for each option:

A. **Only sedimentary rock can undergo heat and pressure to form metamorphic rock.** - This is false because both igneous and sedimentary rocks can undergo metamorphism.

B. **All three types of rock can be formed when magma cools.** - This is false because only igneous rock is formed from cooling magma. Sedimentary and metamorphic rocks are formed through different processes.

C. **Only metamorphic rock can be melted to form magma.** - This is false because both igneous and sedimentary rocks can also be melted to form magma.

D. **All three types of rock can be weathered to form sediment.** - This statement is true because igneous, sedimentary, and metamorphic rocks can all be broken down into sediment through weathering processes.