The correct statement about both unicellular and multicellular organisms is:
Both have DNA.
Unicellular organisms (such as bacteria) and multicellular organisms (such as plants and animals) both possess DNA, which is essential for storing genetic information and guiding various cellular processes. The other statements are not true for both groups: unicellular organisms are made of a single cell, while multicellular organisms are made of many cells; they can be eukaryotic or prokaryotic, but unicellular organisms are primarily prokaryotic, and multicellular organisms are mainly eukaryotic; and they are not the same size, as unicellular organisms are typically much smaller than multicellular organisms.