Aparticular flask has a mass of 17.4916 g when empty. when filled with ordinary water at 20.0°c (density = 0.9982 g/ml), the mass of the flask is now 43.9616 g. the density of so-called "heavy water" at 20.0°c is 1.1053 g/ml. what will the mass of the flask be when filled with heavy water at 20.0°c?

The mass of the empty flask is 17.4916 g. Now after feeling the ordinary water the mass of the flask is 43.9616 g. Thus the change of weight due to addition of ordinary water is (43.9616 - 17.4916) = 26.47 g.

Now as the density of the ordinary water at 20°C is 0.9982 g/ml, so 26.47 g is equivalent to mL of water. Thus the capacity of the flask is 26.5177 mL.

Now the density of heavy water is 1.1053 g/mL at 20°C. Thus 26.5177 mL of heavy water is equivalent to (1.1053×26.5177) = 29.310 g.

Thus the total weight of the flask filled with heavy water will be (17.4916 + 29.310) = 46.8016 g at 20°C.  

there are two significant figures.

c) should be the answer.