Wikipedia lists shallots as an allium used to flavor Pad Thai alongside garlic.
In the dish’s ingredient description, garlic and shallots contribute aromatic depth to the wok-fried noodles. They are part of the savory base that balances the brighter flavors from tamarind, fish sauce, sugar, chili, and lime. Depending on the recipe, the aromatics may be chopped, sliced, or incorporated into a broader sauce mixture.
Scallions are an easy wrong answer because green onions commonly appear as a garnish or vegetable in Pad Thai. They are associated with the finished plate, but Wikipedia specifically names shallots with garlic among the flavoring ingredients. Leeks are not the listed answer, and “onions” is less precise than the ingredient named in the article.
Pad Thai recipes vary considerably by vendor and region. Shrimp, tofu, dried shrimp, bean sprouts, chives, peanuts, and lime may be added or served separately. Vegetarian versions can replace fish sauce and omit seafood, but the balance of aromatic, sour, sweet, salty, and spicy elements remains characteristic.