The subjunctive form has the following structure-
Directive word + that + subject + base formThe base form of a verb is the same as the infinitive form, but without the "to."
A. that the committee’s definition, once agreed upon by a panel of haiku experts,
was thenOnce we strip the non essential part of the sentence, the sentence becomes -
that the committee’s definition,
once agreed upon by a panel of haiku experts,
was then.
We should use the base of the word 'to be'.
B. that the committee’s definition, once agreed upon by a panel of haiku experts, beThis is correct as the correct verb form is used.
C. that the committee’s definition, once agreed upon by a panel of haiku experts,
would bethat the committee’s definition,
once agreed upon by a panel of haiku experts,
would beWe should use the base of the word 'to be'.
D. the committee’s definition, once agreed upon by a panel of haiku experts,
to beThe base form of the verb does not include 'to'
E. the definition of
this committee, once agreed upon by a panel of haiku experts,
was to bewhich committee?
was to be is incorrect here.
Also, the non underlined portion of the sentence has "proposed", hence we should use the subjunctive form in the sentence.