What does the number 18.9984 represent in the image?
a) the atomic number of fluorine
b) the number of protons in fluorine
c) the number of electrons in fluorine
d) the average atomic mass of fluorine

The element symbol in chemistry comprises atomic number and mass number.

Atomic number is written on the top of element and mass number is written at the bottom.

For eg:

If Y is an element that is having atomic number a and mass number x. Then its denotion will be:

ᵃYₓ.

So here ⁹F₁₈.₉₉

Here 18.99 will denote mass number while 9 will denote atomic number.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).