What event leads to Gulliver's departure from Brobdingnag?

Gulliver's departure from Brobdingnag is primarily triggered by his capture by a giant bird. This event occurs after he finds himself in a precarious situation where he is nearly lost to the world of Brobdingnag due to his small size in comparison to the giant inhabitants. The bird, which is a "Roc," picks him up, and this incident ultimately leads to his journey back to the sea and his return to England. This capture serves as a catalyst for his exit from the land of giants, as it forces him to leave rather abruptly. The other options do not accurately reflect the circumstances surrounding his departure, as he does not choose to leave voluntarily, nor does he establish a close friendship that would lead him to stay, nor does he become lost in the wilderness without external intervention. The giant bird's action directly impacts his fate, making this the correct answer.