Final answer:
The appropriate tense to use is the simple present tense, thus the correct answer is 'When we arrive at the forest, it is getting darker.' This suggests that the state of darkness increases as we arrive.
Step-by-step explanation:
The correct verb tenses to use for this sentence are 'arrive' and 'is getting'. When filling the blanks in the sentence, it would be: 'When we arrive at the forest, it is getting darker.' This is a present tense narrative about a current event, where the state of darkness is actively increasing as we are arriving.
The other options 'have arrived / has been getting', 'arrive / has been getting' and 'arrived / was getting' do not correctly match the tense between the two actions. In English, the two actions happening in the same sentence must be in the same tense for it to be grammatically correct.
Learn more about Verb Tenses