Answer is: 12 grams of the isotope carbon-12 (¹²C).
The Avogadro’s number is the number of atoms in 12 grams of the isotope carbon-12 (¹²C).
Na is Avogadro number or Avogadro constant (the number of particles, in this example carbon, that are contained in the amount of substance given by one mole).
The Avogadro number has value 6.022·10²³ 1/mol in the International System of Units; Na = 6.022·10²³ 1/mol.
The answer is C. Because planet A experiences a stronger gravitational field, it will have a larger acceleration due to gravity, and therefor orbit faster than planet B
