The equation for density is mass divided by volume. if the volume of an object increases, but the mass stays the same, the density will

a stay the same
bincrease
c decrease

I believe the density will decrease, because if you look at the equation d=m/v. This can also be written as d=m(1/v). So if you increase volume, let's say you increase from 10ml to 1000ml. And keeping mass same, say 5 grams,then instead of having 1/10= 0.1ml then putting the equation together as 5x0.1. You will have for the second equation 5x(1/1000ml)= 5x0.001. You can see for yourself after multiplying these 2 values from the different equation which density value is smaller and larger.

Wavelength divides in half and speed stays the same.