Which of the following correctly represents an element? A Water - H20
B Copper - Cu
C Carbon monoxide - CO
D Salt - NaCl

A water H2O

Explanation:

H2O is composed of 2 hydrogens which is represented H2 and the hydrogen is combined with one oxygen which forms water which is H2O

when it gains or loses electrons

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.