Royal Mail has the adaptable to set its own prices in order to responding to a rapidly changing market and safeguard the universally postal service.

The first emboldened word must be a noun because it is preceded by definite article 'the'. According to this, option a) and the emboldened word are negated because these are adjectives. The second emboldened word must be a verb in order to connect the noun 'prices' and the noun phrase 'rapidly changing market'. Further, it is preceded by 'to' which indicates that the verb must be used in the base form (infinitive construct: 'to' + base form of the verb). According to this, the emboldened word and option b) are negated because these are present participle form of verbs. The third emboldened word must be an adjective in order to qualify the noun phrase 'postal service'. According to this, the emboldened word and option c) are negated because these are adverbs. Option d) satisfies all the grammatical conditions mentioned above and brings out a meaningfully coherent sentence. Hence, d) is the correct answer.