d) Marginal Probability of event A
Let us say i have two boxes A and B and both have red balls. Now i want to find the probability of Red ball.Now ball can come from box A and box B.These are mutual exclusive events as ball will come from A or B and collectively exhaustive events.So Bayee's theorem will be applicable here,by which we can say that Probability of Event Red ball(Er) is the sum of joint probabilities of selecting a box and getting Red ball.
Now getting Red ball is the Event A as per your question. And occurrence of Some events from question means Selecting box 1 and 2,which is depending on the Event of selecting ball Red.So we get here the marginal probability of A