Final answer:
Measuring the amount of deuterium in the universe provides an estimate of the universe's density of ordinary matter (baryonic matter) at the time when it was approximately 4 minutes old. This density informs us about the current density of baryonic matter in the universe, which falls short of the critical density needed for a closed universe.
Step-by-step explanation:
When we talk about measuring the amount of deuterium in the universe, we are referring to an important observational tool used by astronomers to infer certain characteristics about the early universe, specifically its density at a particular point in time. According to detailed research and astronomical observations, the abundance of deuterium can be linked to the density of ordinary matter (baryonic matter) in the universe when it was about 4 minutes old. The deuterium we see today survived because the density at the time of its formation was low enough; a higher density would have led to greater conversion of deuterium into helium.
The measurements of deuterium restrict us to an estimate of the present-day density of ordinary matter, which is determined by particles such as protons and neutrons. Interestingly enough, the density calculated from deuterium abundance falls short by a factor of about 20 to create a critical-density universe. Therefore, the answer to the student's question would be that the measurement of deuterium allows us to set a limit on C) the density of ordinary (baryonic) matter in the universe.