Which is the best interpretation of the two flat portions of the graph?

In the two flat portions of the graph, the heat supplied does not lead to rise in temperature.

Latent heat is the heat supplied to a body which does not result in a rise in temperature. The word "latent" connotes "hidden".

Thus, at those two portions of the graph, the heat supplied leads to the breaking of the intermolecular bonds of:

Hence, those portions are flat because, the heat supplied does not result in rise in temperature.

For more explanation on latent heat see:

b.final

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.