Consider the terms elements, compounds, and mixtures. What do you think each of
these is?

Explanation:

Element -

It is the basic unit of matter, which cannot be broken down into substances by any way but can be combined to form new substances.

It is a pure substance.

It classifies into metals, non-metals, metalloids and noble gases.

Example - Hydrogen, Carbon, Oxygen

Compounds -

It is a combination of two or more elements.

It is a pure substance.

Through chemical reactions, they can be broken down into metals.

Example - Water, Carbon dioxide

Mixtures -

It is a combination of two or more elements or compounds or both.

The substances combined in a mixture are in a particular ratio.

Example - Chalk powder, Washing Soda

i would say b

explanation:

but i think i would need more information than that

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.