Final answer:
Option D. Spin, grin, twin can be added to the first column without significantly altering the difficulty level as they have similar sounds and structures.
Step-by-step explanation:
In the context of the question, we are asked to identify words that can be added to the first column without significantly altering the difficulty level. Looking at the choices, option D. Spin, grin, twin can be added to the first column without significantly altering the difficulty level. This is because all the words in option D have similar sounds and structures as the words in the first column. Both sets of words contain three letters and share the -in sound.