Apure substance that is made of only one kind of atom is called a. an element b. an electron c. a particle d. a proton

An element is a pure substance made up of only one kind of atom. An element is composed of atoms in which it is composed of neutrons and protons. The nucleus is surrounded by electrons. The answer to this problem is A. element

Among all three PCl₃, NO₃⁻ , I₃⁻, H₂Se only I₃⁻ will involve participation of d-orbitals in hybridization as bonding in I₃⁻ will include, s, p and d orbital as shown in the image attached. there will be sp³d hybridization, there will be presence of three lone pairs and 2 bonds as

I⁻ has 8 valence electrons and 2 neighbours atoms which will need one electron each to satisfy their valency.

so the number of electrons on central atom will be:

8-1-1=6

That 6 electrons will make 6/2 =3 lone pairs.