Desafortunadamente, el actor ya está a. muerta b. muerto c. muertos

Muerto is an adjective that means dead. An adjective is a word that gives us more information about a noun or pronoun. Adjectives must agree with the noun or pronoun they describe in both number and gender. Since actor is a masculine singular noun, then we must choose a feminine singular adjective and muerto meets this requirement. Muerta is a feminine singular adjective and muertos is a masculine plural adjective. So:

Desafortunadamente, el actor ya está muerto