To identify a foreign policy duty of Congress, we need to look for an option that aligns with the responsibilities of the legislative branch in shaping and overseeing the country's foreign relations. Let's analyze each option to determine the correct one.

Option A: Declaring war

Option B: Negotiating treaties

Option C: Appointing ambassadors

Option D: Enforcing immigration laws

Let's break down each option to find the one that represents a foreign policy duty of Congress.

#### Solution By Steps

***Step 1: Declaring War***

Declaring war is a significant foreign policy decision, but the power to declare war is vested in Congress according to the U.S. Constitution. This aligns with Congress's role in foreign policy.

***Step 2: Negotiating Treaties***

While negotiating treaties is a foreign policy function, the President negotiates treaties, but they require Senate approval. This is not a duty exclusive to Congress.

***Step 3: Appointing Ambassadors***

The President appoints ambassadors, but the Senate must confirm these appointments. While Congress has a role in the confirmation process, the initial appointment is made by the President.

***Step 4: Enforcing Immigration Laws***

Enforcing immigration laws falls more within the realm of domestic policy rather than foreign policy.

#### Final Answer

The correct option that illustrates a foreign policy duty of Congress is:

- Option A: Declaring war

Summary:

Congress plays a crucial role in foreign policy by having the power to declare war, a significant foreign policy decision that falls within its constitutional duties. This power ensures that the legislative branch has a check on the country's military engagements and foreign relations, emphasizing the importance of congressional oversight in matters of national security and international affairs.