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B is corrent. This solution includes the variable cost of the 300,000 units produced as well as the $2.50 incremental variable cost for the new direct material.  See supporting calculations.

Supporting calculations 

Original:       

Units produced         $300,000      

 Good units              (88%)   264,000 

Revenue                  264,000 × $5013,200,000 

Production department costs     

264,000 × $29.50       

264,000 × $21.50       

300,000 × $21.50       (6,450,000)

Margin                        6,750,000

New material:       

Units produced             300,000      

Good units (98%)          294,000

Revenue                    

                          294,000 × $5014,700,000 

Production department costs      

 294,000 × $29.50     

  300,000 × $21.50         (6,450,000)      

300,000 × $ 2.50       

294,000 × $21.50       

294,000 × $ 2.50       

30,000 × $  2.50                                 

Margin                          7,500,000

Increase (decrease)          $750,000​

A is incorrect. This solution is the same as the correct solution except for the incremental cost of the new material. In this solution, the incremental cost of the new direct material is included only for the increase in the number of goods produced (294,000 – 264,000 = 30,000) which is not appropriate—the new material cost would be incurred for all 300,000 units.

C is incorrect. This solution only considers the production costs of the good units sold and excludes the scrap cost of the defective units. A full 300,000 units would be produced with direct material so that the cost of the total units produced must be included in the costs. The solution also includes fixed manufacturing overhead which is inappropriate; the fixed manufacturing overhead will be the same regardless of what direct material is used, and thus, it can be omitted from the solution. (Note: The fixed manufacturing overhead is really irrelevant because it will be incurred in full under both situations. The inclusion of fixed manufacturing overhead in response A is inappropriate because this solution is attempting to treat the overhead as a variable cost.)
D is incorrect. This solution only considers the variable costs of the good units produced and excludes the variable costs of the defective units produced. A full 300,000 units would be produced with direct material so that the variable costs of all 300,000 units produced must be included in the costs; this includes the additional $2.50 incremental cost for the new direct material.