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B is corrent. The requirement is to determine the date by which the IRS must assert a notice of deficiency for a 2012 calendar-year return. The normal period for assessment is the later of 3 years after the return is filed, or 3 years after the due date of the return. The assessment period is extended to 6 years if the gross income omitted from the return exceeds 25% of the gross income reported on the return. In this case, since the $45,000 of gross income inadvertently omitted from the return exceeds 25% of the $120,000 of gross income reported on the return, the 6-year period for assessment applies. Since the return was due and also filed on April 15, 2013, a notice of deficiency must be assessed by April 15, 2019.
A is incorrect because the deficiency must be assessed by April 15, 2019.
C is incorrect because the deficiency must be assessed by April 15, 2019. 
D is incorrect because the deficiency must be assessed by April 15, 2019.