The sentence that contains an inadequate shift in the number would be:
D). Some of the students brought his or her own lunch.
Pronoun Shift Error
The pronoun shift error is described as a flaw in which the number of pronouns is inappropriately replaced by a different one in the sentence.
Among the given options, the last sentence i.e. "Some of...lunch" employs this error because the first part of the sentence employs a plural pronoun while the latter inadequately places a singular pronoun i.e. 'his or her."
This makes the sentence grammatical incorrect and affects its meaning and comprehension as well.
Thus, option D is the correct answer.
Learn more about "Shift in Number" here:
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