Which of the following elements has five valence electrons?
a. boron
b. tungsten
c. rubidium
d. phosphorus

D. Phosphorus

You can tell what elements have how many valence electrons by what group they're in. Groups are vertical. 
So group 1 has 1 valence electron 
Group 2 has 2 valence electrons 
Group 3 has 3 valence electrons, etc. 

Moles = mass/molar mass the molar mass of c20 = 12.011*20 = 240.22 g per mole therefore, the number of moles is 20 g / 240.22 g per mol = 0.0833 moles.