If an object's density is decreasing but its mass stays constant, what must be true of the object's volume?

The volume will be increased

Look at the formula

If the mass increase the density will be increased because of their direct relationship. But in the case of constant mass. The volume increase the density will decrease because there is an inverse relationship exists between them. In the inverse relationship, the two objects perform differently. for example, if you are going to inflate the balloon, its volume will increase but density decrease