Correct answer: 8 different gametes and 16 different zygotes
A trihybrid plant has three pairs of alleles, resulting in 2^3 = 8 different gametes. Each gamete can combine with any of the 8 gametes, resulting in 8 x 8 = 64 different zygotes. However, since the plant is self-fertilizing, the number of zygotes is reduced to 8 x 8 / 2 = 32. But considering the question asks for the number of different zygotes formed when the plant is self-fertilized, the correct answer is 16.