In order for oil to be trapped underground and form a reservoir, specific geological structures need to be present. An ideal trap would be a structure that helps to contain the oil and prevent it from leaking or escaping.

Option 1, a horizontal structure, is unlikely to be an ideal trap. This is because the oil could easily migrate and disperse through the porous rocks without being trapped.

Option 2, a fault structure, may or may not be an ideal trap. Faults can create fractures and pathways for oil to migrate, but if these fractures intersect with impermeable layers of rock, the oil could be trapped.

Option 3, a synclinal structure, is unlikely to be an ideal trap. Synclines are downward-folding structures, and oil could potentially migrate along the slopes of these folds, causing it to disperse rather than being trapped.

Option 4, an anticlinal structure, is the correct answer. Anticlines are upward-folding structures, which create a dome-like shape. The convex shape of an anticline can provide a natural barrier for the oil, preventing it from migrating and allowing it to pool and accumulate within the folds of the structure. This makes it an ideal trap for oil.

Therefore, the correct answer is