B is the best choice.Based on parallelism, A, C, and E can be eliminated. The parallelism is set off by the preposition
by and we expect an object in the form of a noun or pronoun to follow a preposition. In A and E, hiring, a simple gerund can only be parallel to another simple gerund. Hence hiring and an increase are not parallel. Similarly in C, a hiring is not parallel to increasing.
Between B and D, D is wordy. The use of increasing the amount of funding in D is wordy and redundant compared with increasing funding in B.
Though some education experts claim that the low test scores of children in the city can be raised by hiring more teachers and an increase in the amount of funding for textbooks and other supplies, others insist that doing this cannot guarantee any improvement.
A.
by hiring more teachers and
an increase in the amount of funding for textbooks and other supplies, others insist that doing this
B.
by hiring more teachers and
increasing funding for textbooks and other supplies, others insist that doing so
C.
by a hiring of more teachers and
increasing the amount of funding for textbooks and other supplies, other insist that doing so
D.
by hiring more teachers and
increasing the amount of funding for textbooks and other supplies, others insist that doing this
E.
by hiring more teachers and
an increase in funding for textbooks and other supplies, others insist that doing so