I found the exercise on the internet and these were the options:
"-Help
-Pelh
-Elhp
-Pleh"
The letter "p" would be fastest recognised by an English speaker in "Help". Letters are more recognisable when they are in a context that's known. If any of the other words was given to an English speaker to memorise, he would probably memorise the letter by letter, while with "help", he would be able to know straight way what letters were contained in it without having to make a conscious effort to memorise them one by one. Because of this, the letter "p", for an English speaker would be the fastest recognised in the word "help" rather then in other unknown word.