The correct answer is: E. No error
The sentence is grammatically correct. The word “so as to” is a subordinating conjunction that introduces a clause that expresses purpose. The word “escape” is a verb that is in the infinitive form. The infinitive form of a verb is used after certain subordinating conjunctions, such as “so as to.”
The sentence can be rewritten as follows:
He fixed a metal ladder for the wall below his window so that he could escape if there was a fire.
The sentence is also grammatically correct in this form. However, the original sentence is more concise and easier to read.