Step 1: Understanding the Concept
After digestion and absorption, glucose, amino acids and fats are carried by blood to all body cells.
Step 2: Identify the process
The cells use these nutrients to build new protoplasm and cell components for growth and repair.
This step of using absorbed food for building up the body is called assimilation.
Step 3: Why the other options fail
Anabolism and catabolism are general terms for building up and breaking down of molecules. Nutrition is the whole process of taking in and using food.
Final Answer:
The process is called assimilation.
\[ \boxed{\text{(B) }\text{Assimilation}} \]