Newton's Third Law: Forces are equal and opposite during collision

Steps:

• The isolated spheres experience only mutual forces, equal in magnitude and opposite in direction (Newton's Third Law).
• Average force on 2.0 kg sphere: F = m₁ × a₁ = 2.0 kg × 5.0 m/s² = 10 N.
• This force magnitude acts on the 4.0 kg sphere.
• Average acceleration of 4.0 kg sphere: a₂ = F / m₂ = 10 N / 4.0 kg = 2.5 m/s² (magnitude; direction opposite).

Why B is correct:

• B (4.0 m/s²) does not match; correct value is 2.5 m/s² from F = ma equality.

Why the others are wrong:

• A (2.0 m/s²): Slightly underestimates the inverse mass ratio (2/4 × 5 = 2.5).
• C (8.0 m/s²): Wrongly assumes direct proportionality to mass ratio.
• D (16 m/s²): Incorrectly doubles the error of C.

Not enough information to select from given choices, as 2.5 m/s² is the precise result.

Final answer: 2.5 m/s²