The difference between an element and a compound is that an element is composed of only one kind of atom while a compound is composed of at least 2 kinds of atoms. The properties of an element and the compound the atoms has to form are different physically and chemically.Example of element are O and H, while compound examples are H2O and H2O2.
I believe the answer is C because the first two are mechanical hazards and C mentions plug which is electrical.
To find them you would have numbers of the elements in percentage or grams then you divide them by their molar mass to get their moles. From there you divide by the smallest number. Round it to two or one sig fig. If you have a number that is for ex. 2.5 you multiply it by 2 to make it whole as well the other whole numbers. Then to find the molecular formula the problem must give you another molar mass and using your empirical formula convert it to its molar mass then you divide them, larger number over smaller number. You should get a number round it to 1 sig fig. Now you use that number and multiply the subscripts on the empirical formula to get the molecular formula.