1. The problem asks which triangles are similar to triangle $\triangle ABC$.\n\n2. Triangles are similar if their corresponding angles are equal and their corresponding sides are proportional.\n\n3. To determine similarity, we compare $\triangle ABC$ with $\triangle DEF$ and $\triangle GHI$ by checking angles and side ratios.\n\n4. If $\triangle DEF$ has the same angles as $\triangle ABC$ and the sides are proportional, then $\triangle DEF$ is similar to $\triangle ABC$.\n\n5. Similarly, check $\triangle GHI$ for angle equality and side proportionality with $\triangle ABC$.\n\n6. Based on the given options, if only $\triangle DEF$ meets these criteria, then only $\triangle DEF$ is similar.\n\n7. If only $\triangle GHI$ meets these criteria, then only $\triangle GHI$ is similar.\n\n8. If both meet the criteria, then both are similar.\n\n9. If neither meets the criteria, then neither is similar.\n\nSince the problem does not provide specific angle or side measures, the answer depends on the given data. Please check the angles and side lengths to conclude similarity.