20.3 g
First, determine how many moles of Li3N you can produce from 1.75 moles of Li. Since each molecule of Li3N requires 3 atoms of Li, that means that from 1.75 moles of Li, you can only produce 1.75 / 3 = 0.5833 moles of Li3N.
Now compute the molar mass of Li3N
Atomic weight of Lithium = 6.941
Atomic weight of Nitrogen = 14.0067
Molar mass of Li3N = 3 * 6.941 + 14.0067 = 34.8297 g/mol
Now multiply the molar mass by the number of moles
34.8297 * 0.5833 = 20.316 g
Of the available 5 choices, only 20.3 g is correct.