Cover of
4. Assess the importance of variations to
natural selection. SC.7.L.15.2

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.

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