Which of the following shows an example of unbalanced forces acting on one or more objects? A- a person sitting still in a chair
B-two cards leaning against each other
C-one football player pushing another down to the ground
D-two football players pushing each other without either one of them moving

B

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.