Question

A run-on sentence is

missing a subject
missing a verb
a writing error
an incomplete thought

Answer 1

A writing error--- Run-on sentences are sentences that keep going and going, they don't have correct punctuation and such. ( For example: I like to play in the sandbox and slide and the swings and the etc...)  

Answer 2

Answer: a writing error .

Explanation: A run-on sentence is basically a writing error which occurs when multiple (two or more) complete sentences or independent clauses are connected improperly. Comma splice is also a common type of run-on sentence which occurs when two independent clauses are connected using just a comma. This error can be corrected by connecting or separating the clauses or sentences into its parts correctly.