Final answer:
The platy, parallel mineral grains that are the most visual aspect of foliated metamorphic rock are micas.
Step-by-step explanation:
The most visible aspect of foliated metamorphic rock is the presence of platy, parallel mineral grains which often contribute to the rock's ability to split along those parallel planes. These platy minerals are typically micas, which include minerals such as biotite and muscovite. Other minerals like quartz, feldspar, and carbonates do occur in metamorphic rocks but do not usually impart the foliated texture that is characteristic of these types of rocks.