Choice B is the correct answer. 'Halved' means the factor is $1/2$ or 0.5.

1. Identify Factor: Halving means multiplying by 0.5 each step. So $b=0.5$.
2. Identify Initial: Initial mass is 64, so $a=64$.
3. Form Equation: $M(t) = 64(0.5)^t$.

💡 Strategic Tip: Half-life problems always have a base of $1/2$ or $0.5$ (unless written with $e$).

Choice A is incorrect because base 2 means doubling. Choice C is incorrect because initial mass is 64, not 32. Choice D is incorrect because base 1.5 means growing by 50%.