You should define the default intercompany balancing rule at the legal entity level, because each ledger has one legal entity assigned to it and each legal entity is associated with one balancing segment value. This way, you can specify the intercompany receivables and payables accounts for each legal entity and ensure that the journals are balanced by legal entity or primary balancing segment values. A ledger-level rule would apply to all legal entities in the ledger, which may not be appropriate if they have different intercompany accounts. A primary balancing segment rule would apply to all ledgers that share the same chart of accounts, which may not be desirable if they have different intercompany rules. A chart of accounts rule would apply to all ledgers and legal entities that use the same chart of accounts, which may not be feasible if they have different intercompany segments or accounts. References:
* Intercompany Balancing Rules, Section: Define Intercompany Balancing Rules
* Overview of Intercompany Balancing Rules, Section: Define Intercompany Balancing Rules
* Troubleshooting Guide For Intercompany Balancing, Section: 1. Journals and subledger accounting entries are not being balanced for intercompany activity
* Implement General Ledger, Section: Intercompany Balancing

According to Oracle documentation1, the most efficient way to update your existing cross-validation rules when you have redesigned your chart of accounts is to use Cross-Validation Rules Import file-based data import (FBDI). FBDI enables you to import cross-validation rules from a spreadsheet template into General Ledger. You can use FBDI to create new rules, update existing rules, or delete rules. Therefore, option C is correct. Option A is incorrect because using the Manage General Ledger Security page does not enable you to update cross-validation rules. Option B is incorrect because creating Cross-Validation Rules desktop- integrated spreadsheet does not enable you to update cross-validation rules. Option D is incorrect because using the Manage Cross-Validation Rules page does not enable you to update cross-validation rules efficiently.
According to Oracle documentation2, you should create a Data Access Set that allows access to the UK Ledger to allow users with the General Accountant job role to access the UK Ledger. A Data Access Set is a security feature that defines the ledgers and balancing segment values that a user can access. You can assign Data Access Sets to users or roles using the Manage Data Access for Users page. Therefore, option B is correct. Option A is incorrect because assigning the security context value of UK Ledger to the user/role combination does not enable access to the ledger. Option C is incorrect because assigning the General Accounting Manager role to those users does not enable access to the ledger. Option D is incorrect because assigning the UK reference set to the user/role combination does not enable access to the ledger.