ANSWER:
a. Water vapor
Explanation:
The density of water vapor is less than the density of a water liquid, in the case of ice, we can see how ice floats on liquid water, which indicates that it has a lower density.
The water obtains its highest density at 4°C, being equal to 1000 kg/m³, in that state it is still liquid water, at room temperature, it is a little lower but it is still a higher density than water vapor, boiling water and ice.
Water vapor would have a lower density than all of them since it is a gas, so the correct answer is a. Water vapor