Final answer:
The run-on sentence is 'Some people like to use shaving cream others use a gel instead,' as it contains two complete sentences not properly separated by punctuation.
Step-by-step explanation:
The run-on sentence from the options provided is: "Some people like to use shaving cream others use a gel instead." This sentence has two complete thoughts without proper punctuation to separate them.
Run-on sentences can make your writing difficult to understand and are seen as grammatical errors. They can be corrected by using a coordinating conjunction, placing a period between independent clauses, using a semicolon, or turning one clause into a subordinate clause with a subordinating word.