Answer: This is a complex and complete sentence.
Step-by-step explanation:
Complex sentences contain an independent clause and at least one dependent clause. While an independent clause expresses a complete thought and can stand alone as a sentence, dependent clause cannot stand on its own, even when it contains both the subject and the verb.
In this particular sentence, we have a dependent clause- 'looking on with horror'. It is dependent because it cannot stand on its own - it just serves as an introduction to the independent clause - 'the two girls saw the tornado approaching.'
*Note that this sentence is also complete, as it begins with a capital letter, has one independent clause and ends with an end mark '.'