Which organisms have groups of cells arranged into organs

The correct answer is multicellular organisms.

Explanation:

Organs are differentiated structure formed by group of cells and tissues to perform the similar functions. When more than two organs perform specific function in a coordinated manner they form organ system.

Tissues and organs are a characteristic feature of multicellular organisms as more than one cell is present in these organisms and these cells aggregate together to form tissues, which further aggregate to form different organs carrying out the different functions attaining complexity.

Since, unicellular organisms do not have more than one cell, unicellular cells do not form tissues and lack organs.

Thus, the correct answer is multicellular organisms.

 which is absent in unicellular organisms.

oxygen +1 most likely