Which element in period 3 on the periodic table is a metalloid? magnesium (mg) aluminum (al) silicon (si) sulfur (s)

Chemical elements exhibit chemical as well as physical properties lie between metals and nonmetals is known as mettaloids.

Metalloids are present on the right hand side of the periodic table such as  boron, silicon, germanium, arsenic, antimony, and tellurium. The series of these six elements is said to be metalloids.

In the attached image, six elements are present in the right side and in grey color are metalloids.  Now, the metalloid which is present in period (from left to right) three on the periodic table is silicon.

Thus, silicon is the correct answer.

where's the diagram?

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