Step 1: Look at what each option depended on.
Ireland's poor were living almost entirely on potatoes as their daily food, so a crop failure there meant real disaster, unlike France, Sweden or the US where diets were more varied.
Step 2: Recall the blight itself.
A fungal disease wiped out potato crops in the 1840s, and because Ireland had no backup food source, the effect was catastrophic, over a million deaths and mass emigration.
Step 3: Match this to the question.
Since the devastation described matches Ireland's dependence on the potato, that is the country being asked about.
Hence, the correct answer is Ireland.