The most challenging factor to consider in merging the two Salesforce.com orgs into a single org is the customer data, as it may contain duplicates, inconsistencies, and conflicts. Option D suggests using standardization and de-duplication tools to merge the customer data, which is a reasonable approach. Option A is not true, as transactional sales data may have different formats, fields, and values in the two orgs. Option B is not necessary, as salespersons selling both product lines can use a single login in the merged org. Option C is not true, as business processes on standard objects may have different workflows, validation rules, and triggers in the two orgs.