Comi el platano means - I at a banana. So, here, you want to get rid of the object, and instead of that use the pronoun to replace it. This is the correct answer then: B. Lo comi. You have to use lo because el platano is signular, masculine, which means that C and D are incorrect, because las is plural, feminine. This leaves us with A and B. A is incorrect because the direct object comes before the verb in Spanish, so B has to be the correct answer.
