Answer : The correct option is, the periodic table.
Explanation :
As we know that the periodic table tell us about many information of the elements like, the name of element, the symbol of element, atomic number, atomic mass, number of protons, number of electrons, the group number, the period number, the number of valence electrons, the energy shell, the element is metal or non-metal, etc.
For example : 'Na' is a metal which belongs to group 1 and period 3. The name of 'Na' metal is sodium and the atomic number 11, atomic mass 23.
The electronic configuration of 'Na' is,
. The number of valence electrons in 'Na' is, 1.
As per question, when Gloria is writing the chemical formula of a compound by using its chemical name. She has just identified the names of the elements in the compound. So, she will use the periodic table because all the informations are given in periodic table.
Hence, the correct option is, the periodic table.