Answer: I. It is a physical change.
Explanation: Physical change is defined as the change which can be observed through naked eyes. This property is known as Physical Property and the substances which follow this property are known as physical substances.
In this case, solid butter changes its state from solid to liquid by melting itself due to the temperature in the microwave. Thus it does not make a new substance and it does not quality as the chemical change as it does not changing its chemical identity. Thus it is a physical change.