Differentiated cells will first organize themselves into
a. a new organism
b. organ systems
c. organs
d. tissues

The correct answer is option D. "Tissues".

Explanation:

During embryogenesis, the fertilized egg divides into multiple undifferentiated cell cells, which later starts to differentiate and specialize. Differentiated cells first organize themselves into tissues, later in organs and systems. For instance, the endoderm, mesoderm, and ectoderm are formed during gastrulation. Later, the mesoderm differentiates into the muscle, skeletal and connective tissue, before the nervous system and the organs are formed.