Which of the following statements best describes why a solution of 6.00 g of ca(no3)2? in 30.0 g of water has a greater boiling-point elevation than a solution of 3.00 g of ca(no3)2? in 60.0 g of water?

Boiling-point elevation is a colligative property.

That means, the the boiling-point elevation depends on the molar content (fraction) of solute.

The dependency is ΔTb = Kb*m

Where ΔTb is the elevation in the boiling point, kb is the boiling constant, and m is the molality.

A solution of 6.00 g of Ca(NO3) in 30.0 g of water has 4 times the molal concentration of a solution of 3.00 g of Ca(NO3)2 in 60.0 g of water.:

(6.00g/molar mass) / 0.030kg = 200 /molar mass
(3.00g/molar mass) / 0.060kg =   50/molar mass

=> 200 / 50 = 4.

Then, given the direct proportion of the elevation of the boiling point with the molal concentration, the solution of 6.00 g  of CaNO3 in 30 g of water will exhibit a greater boiling point elevation.

Or, what is the same, the solution with higher molality will have the higher boiling point.