Answer:
The student incorrectly simplified  .
 .
Thus, option D is correct.
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Step-by-step explanation:
The formula to determine the Perimeter of a rectangle of width w and length l is expressed as:

In other words, the perimeter can be determined by multiplying the length and width by 2 and adding the result.
In our case, the dimensions of a rectangle are  and
  and   .
.
Here is the student's solution:

                                  
                                   
           
The student made an error in calculating  , because
 , because  are not like terms.
 are not like terms. 
Hence,  can not be simplified to
 can not be simplified to 
Therefore, the student incorrectly simplified  .
 .
Thus, option D is correct.
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<u>Here is the correct Solution:</u>

                                  