Option d: Mia make an error in Step 4
Step-by-step explanation:
The polynomial is

Step 1: Mia replaced
instead of

Thus, the polynomial becomes,

Step 2: Grouping the terms, we have,

Step 3: Factoring the terms, we have,

Step 4: Taking out common terms,

But, Mia made a mistake while taking out of the terms and wrongly written it as

Thus, Mia made a mistake in the 4th step.
Hence, Option d is the correct answer.