How many moles of cobalt (co) atoms are there in
6.00x10^9 (6 billion) co atoms?

9.9vogadro's number represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. This number can be used to convert the number of atoms or molecules into number of moles. Calculation are as follows:

 6.00x10^9 Co atoms x (  1 mol Co / 6.022 x 10^23 atoms Co ) = 9.96 x 10^-15 mol Co

Sorry i think the answer is b