I believe the probability is 3/16.
This is because in classical Mendelian Genetics, each trait can either be dominant or recessive. When two true breeding parents are crossed, the recessive trait is hidden in the offspring, only to reveal itself later in the grandchildren. The classic ratio for a monohybrid cross with straight dominance is 3:1, dominant to recessive in the F2, while the classic ratio for a dihybrid with straight is 9:3:3:1 in the F2. In our case out of this combination there are three offsprings out of the 16 which are have white axial flowers (recessive). Therefore that gives us a probability of 3/16.