Psalm.
A numbered musical piece might be called a concerto, an opus, or a composition. It could also be called a symphony.
The term for a second melody that accompanies the main melody in a musical composition is called a "counterpoint."
Study
instrumental
Usually, they are called a musician.
A group of Musical Instruments is called a consort.
A written composition is usually referred to as an essay, article, or piece of writing. It can also be called a composition or text, depending on the context and format.
Basso continuo
I think it is a motive.
Two singers are a duo.They might sing a duet, a musical composition for two performers.
It's called an etude - i.e. A musical composition written solely to improve technique.
The type of weathering that does not alter the chemical composition of the rock is called physical weathering. The acid weathering usually alter the chemical composition of a rock.