Final answer:
The correct statement is that in the gametophyte stage, haploid gametes are produced, while in the sporophyte stage, diploid spores are produced. This alternation of generations is a key aspect of the plant lifecycle.
Step-by-step explanation:
The two-stage lifecycle of plants consists of the gametophyte stage and the sporophyte stage. In the gametophyte stage, haploid gametes are produced by mitosis within multicellular structures. These gametes then fuse to form a diploid zygote, which develops into the sporophyte. The mature sporophyte stage is diploid and produces haploid spores by meiosis. These spores grow into new gametophytes, completing the cycle. Therefore, the correct statement is: 'In the gametophyte stage, haploid gametes are produced; in the sporophyte stage, diploid spores are produced.'