All of the following became colonial territories except
a. laos.
b. burma.
c. vietnam.
d. thailand.

Laos and Vietnam were part of French Indochina, which was a French colony. (Laos was added to the colony later, in 1893)

Burma was a British colony until 1948 - it was called British Burma. It was  a province of British India.

The correct answer is the remaining option: Thailand