What are the units of the coefficient of friction?

Explanation:

Coefficient of friction is a constant ratio of two forces acting on a body at same time.

This ratio is represented by a letter μ. Suppose forces acting on a body due to its surface and due to pressure applied on it be F and L respectively.

Now as μ = F/L and both F and L are measured in term of force, then μ is a dimensionless quantity and do not have any unit.

there is no correct list among the choices you've provided.

the list might look something like this:

-- baby human

-- piccolo

-- flute

-- soprano singer

-- trumpet

-- tenor singer

-- trombone

-- baritone singer

-- baritone horn

-- bass singer

-- tuba

since i had to make up the list myself, i made it easy on myself, and i didn't try to shuffle the string instruments and woodwinds into it too.