Question

vietnam was divided at the into communist north and non - communist south vietnam.
the united states used the to justify its intervention in southeast asia.
the united states helped c, which was designed to be a military alliance to prevent the spread of communism.
17th parallel
38th parallel
46th parallel

Explanation:

For the first blank (Vietnam's division): Vietnam was divided at the 17th parallel into Communist north and non - Communist south. The 38th parallel is related to the division of Korea, and the 46th parallel is not related to Vietnam's division.
For the second blank (US intervention justification): The domino theory was used by the US to justify intervention in Southeast Asia (though the options here are parallels, maybe a typo, but based on the parallel options and Vietnam, 17th parallel contextually fits the division part which is linked to the intervention's backdrop).
For the third blank (military alliance against communism): SEATO (Southeast Asia Treaty Organization) was helped by the US, and the division at 17th parallel is linked to Vietnam, which is in Southeast Asia. The 17th parallel is the key here for Vietnam's division, so the correct parallel for the first (and related) blanks is 17th parallel.

Answer:

1. 17th parallel (for the first dropdown related to Vietnam's division)
2. 17th parallel (for the second dropdown related to US intervention in Southeast Asia, considering the Vietnam - related context)
3. 17th parallel (for the third dropdown related to the military alliance in Southeast Asia, linked to Vietnam's situation)