activated RNA nucleotides?

Your question is not very clear. What do you mean activated?

They might need to be "primed" with energy from an ATP molecule so that they have enough energy to for a bond with other RNA nucleotides.
When a bond is made, it either takes in energy or releases it. Normally bonds which require an input of energy to form won't formed spontaneously, so the molecules need to be "activated"/"primed"
Hope this helps

Nucleotides can exist in the mono, di or tri phosphorylated state. Also, they're constantly recycled within the cell. Phosphate groups are transferred or lost during a reaction. So an enzyme most likely adds on the phosphate groups which are then lost when two nucleotides are linked together by RNA polymerase.

Yup. mRNA is singke stranded so that's the only way in which the nucleotides are bound together.

In the book that I read, it said that the two phosphoryl groups were added to the bases and they were then said to be activated. The RNA bases would hydrogen bond to the template strand during transcription and the phosphoryl groups were lost providing the energy for RNA Polymerase to catalyse the reaction forming the 'backbone' for the mRNA strand (those phosphodiester bonds).