It depends on the objects chemical composition.
Your answer is correct: Resident 4 would provide the scientist with the most valid results.
Answer:
r = √(k q₁ q₂ / F)
Explanation:
F = k q₁ q₂ / r²
Multiply both sides by r²:
F r² = k q₁ q₂
Divide both sides by F:
r² = k q₁ q₂ / F
Take the square root of both sides:
r = √(k q₁ q₂ / F)
A reasonable hypothesis would be that, nothing changes with can A (no expansion at all), and Can C is expanding, but Can B is expanding even faster than Can C. (I don't know, if this hypothesis is correct though)