Think about it. Etymology understand is very important in instances like this. Cell specialization is no, hence the word "specialization", which means focusing in one area. Unicellular is a no, hence the word "uni" which means one. Organisms that are unicellular just have one cell or a combination of the same cell.
I narrowed it down, now do your own research.

you sure about that?

To quote your definition Darwin: "which means focusing in one area"

Nerve cells seem to be specialized in that they transmit and receive signals, they do not also function as skin cells. Blood cells carry oxygen, but do not sense pressure. Bone cells provide structural support, but do not make ATP from light energy. That sounds specialized to me…
All of these cells can be found in various multicellular organisms.

Yes, cells specialization means doing only a certain job. The nature of this job would have to depend on the density of organelles inside the cells. For example, the palisade cells in plants contain abundant chloroplasts that enable the cells to carry out their job well. This is specialization to me.

multicellular is the answr mate

No, it’s not. Multicellular means "having many cells"