Answer:
Just in 1 dimension
Vpg = Vps + Vsg
Where
Vpg= relative to the ground
Vps= person relative to the ship
Vsg= ship relative to the ground
Vpg= Vps+Vsg ;
Vpg= 0.5 + 6
Vpg= 6,5 m/s
A is correct according to below calculation.
m₁v₀₁+m₂v₀₂=m₁v₁+m₂v₂
((m₁v₀₁+m₂v₀₂)-m₁v₁)/m₂=v₂
v₂=((.5*12-.75*16)+(.5*21.6))/.75
v₂=6.4 m/s
As we know,

so, let's solve for velocity (v)
A kilogram is a unit of weight. So a kilogram of bricks would weigh the same as a kilogram of feathers despite if its in water or air since weight is determined by gravity in relation to mass and not what substance the object is in.
That the hydrogen atom was based on the Quantum theory