The group number is the same as the number of the valence electrons each element within that group has. Therefore, two elements in the same group will have the same number of valence electrons. Since all bonding and/or ionizations involve the valence shell of electrons, atoms with the same number of valence electrons behave similarly. Both tellurium and sulfur are in group VI, and have 6 valence electrons.

The period number/row number is the energy level for each element. Therefore, two atoms found in the same row on the periodic table have the same energy level. Potassium and selenium are both in row 4 of the periodic table. Their highest energy level for electrons is 4.

Ionization energy is the energy required to remove an electron. Noble gases are special because the have a full valence shell of electrons, which makes them the most stable elements, and to remove an electron requires a lot of energy. The trend for ionization energy is as follows: ionization energy increases from left to right within a row and from bottom to top within a group on the periodic table. All of the answer choices are in the same row of the periodic table, but fluorine is the furthest to the right. Thus fluorine has the greatest ionization energy.

For neutral atoms, the number of valence electrons is equal to the atom's main group number. According to the periodic table, oxygen is in group 6; therefore, it has 6 valence electrons in its outer shell.

Generally speaking, as you go across a period and up a group on the periodic table, electronegativity increases. Fluorine is the most electronegative element, with a Pauling scale electronegativity ranking of approximately 4.0.

Atomic radii increases from right to left of the periodic table and it decreases bottom to top. So francium, in the bottom left of the periodic table, has the largest atomic radius; helium, in the top right of the chart, has the smallest atomic radius. Based on these trends, cesium would be least likely to hold on to its valence electrons because it has a larger atomic radius compared to cobalt or iron. Compare this to helium which has a small atomic radius and a full valence shell of electrons, which makes it very stable.

Halogens would require the greatest first ionization energy to dislodge one of their valence shell electrons because they have both the greatest electron affinity and the smallest atomic radii. Since their electrons are both closer to their nuclei and halogens are more "electron greedy" (electronegative), they require more energy to remove an electron. However, since the noble gasses have full valence shells, they have the greatest first ionization energies.

Atomic radii decrease from left to right across the periodic table and increase from top to bottom of the periodic table. Based on these trends, helium has the smallest atomic radii and more atoms would fit inside our imaginary container. 

Electronegativity is the tendency of an atom to attract electrons to it. Electronegative atoms are electron "greedy". When they form covalent bonds, highly electronegative atoms often form polar covalent bonds in which the electrons spend a greater amount of time near the electronegative atom resulting in a dipole moment. Water is the quintessential example of a polar molecule. 

Electronegativity increases from left to right across the periodic table and increases from bottom to top as well. Fluorine is the most electronegative atom and it would be most likely to result in a polar molecule. Keep in mind that the formation of a polar bond depends on the differing electronegativities of the atoms in question. For example, two oxygen atoms do not make a polar bond even though both atoms are highly electronegative.

Within a row, atomic radius increases as elements go from right to left on the periodic table. There are more proton in the rightmost elements which pulls the electrons closer into the nucleus due to their opposite (attractive) charges. Atomic radius increases from top to bottom within a group because new energy shells are added, which makes the radius larger by definition.