Final answer:
A fast-moving mountain stream carrying well-rounded pebbles, cobbles, and boulders would create a conglomerate sedimentary rock upon lithification, as it is composed of cemented gravel-sized particles.
Step-by-step explanation:
A mountain stream carrying well-rounded pebbles, cobbles, and boulders and then lithifying them would create a conglomerate rock. Conglomerate is a type of clastic sedimentary rock that is composed of significant portions of rounded gravel-sized particles. These particles are typically cemented together by finer material and are indicative of energetic water flow that can transport and deposit such large grains.
The other options provided, such as siltstone, sandstone, and shale, represent sedimentary rocks composed of much finer materials. Siltstone is made from silt-sized particles, sandstone is made from sand-sized particles, and shale is composed of clay-sized particles, often with fine laminations and organic matter like plankton fossils.
Therefore, among the options, conglomerate is the correct answer as it is the rock type that forms from the cementation of gravel-sized particles typical of a fast-flowing mountain stream environment.