Final answer:
The statement is false because not all organisms have adults that are diploid and only gametes that are haploid; fungi and some algae have haploid-dominant life cycles, and plants have a life cycle that alternates between haploid and diploid stages.
Step-by-step explanation:
The statement that 'in all life cycles, the adult organisms are diploid and only the gametes haploid' is false since there are exceptions. In fact, while most animals exhibit a diploid-dominant life cycle, where the adults are diploid and gametes are the only haploid cells, not all organisms follow this pattern.
There are life cycles other than the diploid-dominant, such as the haploid-dominant life cycle found in fungi and some algae, where the multicellular haploid stage is predominant, and the alternation of generations found in plants and some algae, where both haploid and diploid stages are significant.