Ladasha placed a marshmallow in a syringe for science class when she pulled the plunger out increasing the volume in the syringe the marshmallow expanded. how can this best be explained?

Answer is: As the pressure decreased in the syringe the volume of the marshmallow increased.

Boyle's Law (the pressure volume law): volume of a given amount of gas held at constant temperature varies inversely with the applied pressure when the temperature and mass are constant.  

p₁V₁ = p₂V₂ (the product of the initial volume and pressure is equal to the product of the volume and pressure after a change).

When pressure goes up, volume goes down.  

When volume goes up, pressure goes down.