How would you prepare a 1 L solution of 3 M MgO?

Put 3 grams of MgO in the beaker and add enough water to reach the 1 L mark.

Put 120 grams of MgO in the beaker and add exactly 1 L of water.

Put 120 grams of MgO in the beaker and add enough water to reach the 1 L mark.

Put 3 grams of MgO in the beaker and add exactly 1 L of water.