In relation to the pronouns shown in bold in the text above,...

Acho que "that" se refere a language, mas não a "language" da linha 20, mas, sim, a da linha 16.

That se refere a idea de persuasão na linguagem diplomática, e não somente a linguagem em si . Para acertar a questão é necessário ler e analisar todo o texto, não somento o trecho .

Atenção para a parte " The languange of diplomacy, often like poetry, has the ability to move people from mood to mood" [...] ordinary language or THAT of diplomacy - THAT : capacidade de persuasão descrita no trecho - has a momentum and an inner driving forces that is ageless.

e-

That refers in line 21 to driving force; had 'that' referred to momentum as well, the verb should have been in its plural form. a momentum and an inner driving force that are ageless.

The pronoun “that” (l.21) refers to “language” (l.20).

Correct answer:

The pronoun “that” (l.21) refers to “language” (l.16) [The language of diplomacy]