The question is about assigning the appropriate Cloud Spanner IAM role to new users who should have read-only access to the database. Option A, 'roles/spanner.databaseReader', is the correct choice because it grants read-only access to a Cloud Spanner database, which aligns with the requirement for new users. Option B, 'roles/spanner.databaseUser', grants both read and write access, which exceeds the requirement. Option C, 'roles/spanner.viewer', allows viewing of Spanner instances but does not grant access to the data within the databases, making it insufficient for the requirement. Option D, 'roles/spanner.backupWriter', is unrelated to the requirement as it pertains to backup operations, not data access. Therefore, the correct answer is A.