Which of the following sentences is grammatically INCORRECT?

“I work at the hospital but my boyfriend works at the clinic” is grammatically incorrect because it is a run-on sentence.
Run-on sentences are when two or more complete sentences are written as one sentence. Run-on sentences can be fixed by adding a dash (–), a semicolon (;), or a colon (:) between the two sentences. You could also add coordinating conjunction with a comma (, but) or make them two separate sentences with a period (.)
These would be correct:
I work at the hospital – my boyfriend works at the clinic.
I work at the hospital; my boyfriend works at the clinic.
I work at the hospital: my boyfriend works at the clinic.
I work at the hospital, but my boyfriend works at the clinic.
I work at the hospital. My boyfriend works at the clinic.