Final answer:
The most appropriate word to describe the style of the sentence regarding honeybee decline is 'concerned', reflecting a serious and urgent tone.
Step-by-step explanation:
Considering the context provided and the tone of the sentence, the word that best describes the style of the sentence 'The decline in honeybees is a growing concern among beekeepers, agriculturalists, and scientists' is c. concerned. The sentence conveys a serious tone about the issue at hand without leaning towards a casual or highly emotional expression, thereby eliminating options 'b. formal', 'a. excited', and 'd. informal'. The use of the word 'concern' directly signifies a sense of urgency or worry about the decline in honeybees, which is also supported by the content provided regarding the significance of bees in pollination and the consequential effects on ecosystems and agriculture.