Step 1: Background.
Tonkin Free School was established in 1907 in Hanoi, Vietnam, during French colonial rule. It aimed at modern education and promoting nationalism.
Step 2: Options analysis.
(A) Cambodia: Incorrect.
(B) France: Incorrect.
(C) Laos: Incorrect.
(D) Vietnam: Correct — it was established in Vietnam.
Step 3: Conclusion.
Thus, Tonkin Free School was established in Vietnam.