The sentence you provided is an example of a third conditional sentence, which is used to talk about a situation that did not happen in the past and its hypothetical result.

The structure of a third conditional sentence is:
- If + past perfect, ... would have + past participle.

So, the correct way to complete the sentence is:
"If he had been looking in front of himself, he would not have walked into the wall."

Therefore, the completed sentence is:
"If he had been looking in front of himself, he would not have walked into the wall."