Where goods are invoiced by a head office to its branch at cost-plus, the Branch Inventory Adjustment Account is used to determine?

Branch gross profit/loss.

Explanation:
In branch accounting, when goods are invoiced at cost-plus, the Branch Inventory Adjustment Account is utilized to calculate the gross profit or loss of the branch. This adjustment ensures that the branch accounts reflect the correct profit margin and that the pricing difference (cost-plus) between the head office and branch is accurately represented in financial reporting.