The formula mass of a compound is calculated by adding up the atomic masses of all the atoms in the formula. In the case of anhydrous sodium carbonate, the formula is Na2CO3.

To calculate the formula mass, we need to know the atomic masses of the elements involved. The atomic masses of sodium, carbon, and oxygen are given as 23 u, 12 u, and 16 u respectively.

Next, we count the number of each type of atom in the formula. In this case, there are 2 sodium atoms, 1 carbon atom, and 3 oxygen atoms.

To calculate the formula mass, we multiply the atomic mass of each element by the number of atoms of that element, and then add them all up.

For anhydrous sodium carbonate, the formula mass can be calculated as:

(2 x 23 u) + (1 x 12 u) + (3 x 16 u)

= 46 u + 12 u + 48 u

= 106 u

Therefore, the correct answer is option 2: 106 u.