For two dices the total number of possible outcomes is equal to 6*6 = 36. For the numbers' sum to be equal to 6 the number has to be (1,5), (2,4), (3,3), (4,2), (5,1) which shows that the number of times the event happening is equal to 5. The probability of two dices landing on numbers that when sum up is equal to 6 is 5/36. The answer is D.