Step 1: How oil and gas gather.
Hydrocarbons form from buried organic matter and then rise until a seal stops them. They pool only where a trap holds them in place.
Step 2: Kinds of traps.
Traps can be structural, like folds, or stratigraphic, where a change in rock type seals the flow. The question wants the best general answer.
Step 3: Why stratigraphic traps fit.
Stratigraphic traps form when a permeable bed passes into a tight bed, pinching off the oil and gas. They are very common and very effective.
Step 4: Weigh the others.
Anticlines do trap oil, but synclinal troughs are poor traps, and delta bars are more about source than trapping. The broad correct answer here is stratigraphic traps.
Step 5: Final choice.
So accumulation is expected in stratigraphic traps.
\[ \boxed{\text{in stratigraphic traps}} \]