Step 1: Read the question.
We must define what adaptation means in inclusive education.
Step 2: Recall the concept.
Adaptation is the systematic modification of teaching and learning to meet diverse learner needs.
Step 3: Test option A.
Changing only the textbook is far too narrow, so it is wrong.
Step 4: Test option B.
Teaching only in special schools promotes segregation, against inclusion, so it is wrong.
Step 5: Test option D.
Seating students separately leads to isolation, the opposite of inclusion, so it is wrong.
Step 6: Confirm option C.
Modifying assessment, materials, curriculum, or classroom environment covers the full, holistic adaptation.
Step 7: Conclude.
Hence option C is correct.
\[ \boxed{\text{C. Modifying assessment, materials, curriculum, or classroom environment}} \]