Final answer:
CpG islands are located in promoter regions and have low methylation levels, while methylation canyons/valleys are larger regions with low levels of methylation associated with regulatory elements of developmental genes. The correct option is a.
Step-by-step explanation:
The main difference between CpG islands and methylation canyons/valleys is that CpG islands are typically located in promoter regions of genes with a high frequency of CG pairs and usually show low methylation levels, while methylation canyons or valleys represent large-scale regions of the genome that have particularly low levels of CpG methylation and can span large portions of genomic DNA.
More precisely, the correct statement about these genomic regions is that CpG islands contain < 10% average methylation (a. CpG islands are longer and contain < 10% average methylation).
Contrastingly, methylation canyons, also known as valleys, are characterized by being areas in the genome that are hypomethylated and are associated with regulatory elements of key developmental genes. The correct option is a.