The sentence below contains an error. Choose the option that best corrects it.

The documentary was so inform that viewers struggled to stay engaged, despite its visually appealing cinematography.

The original sentence contains grammatical errors. "Inform" is a verb, but an adjective "informative" is needed to describe the documentary. "Engage" should be "engaged" to describe the viewers' state. "Visual appealing" should be "visually appealing," using an adverb to modify the adjective.

Option 1 uses "were" instead of "was" for a singular subject, and retains errors in "engage" and "visual appealing."

Option 2 uses the incorrect adverb "informatively" and changes the tense of "struggle."

Option 3 incorrectly uses "inform" and "engaging."

Option 4 correctly uses "informative" (adjective), "engaged" (past participle as adjective), and "visually appealing" (adverb modifying adjective), correcting all original errors.