Final answer:
The sentence with an informal tone is 'b. The chance meeting was unusual, but also cool because it provided us all a chance to talk.', as it uses casual language that fits everyday speech.
Step-by-step explanation:
The sentence that uses the appropriate word choices to maintain an informal tone is: b. The chance meeting was unusual, but also cool because it provided us all a chance to talk. This sentence is more conversational and uses informal language such as 'cool' and 'us all', which are characteristic of a more relaxed and friendly tone. The use of more casual language, along with a less structured feel, fits better into everyday speech and informal writing. In contrast, other options are more formal and distant, using either neutral, generic terms (option a), expressing sentiment in a more impersonal way (option c), or presenting a stiffer and overly formal choice of words (option d).