Simply correlate with the information that we have. This is not the kind of question where overthinking is required.
We know that these tubules have "an inside diameter equal to their length, a density half that of water, and a mass that is immeasurably small."
So basically- Diameter, density and mass.
*Note- there's no way of knowing what the mass is, only that its very miniscule.
Now, the info of the 'entity' found is that it is "less dense than water, with insufficient mass to measure, and a length exactly equal to the diameter of its inside."
Right off the bat, mass checks off and density is deliberately left ambiguous. The only information that we are provided without any doubt is that of the diameter. Therefore, we must choose the option that attacks this condition precisely, that is, B.
The reason why I mentioned the do-not-overthink disclaimer is because the ambiguity of the density would lead to many people dismissing the option but we are not required to read between every line in this case.