The common size statements are useful, both, in intra-firm comparisons over different years and also in making interfirm comparisons for several years. This analysis is also known as

The correct option is 'Vertical analysis'.

Explanation:
Common-size financial statements present each item on the statement as a percentage of a common base figure (such as total assets for a balance sheet or net sales for an income statement). Because this method analyzes the relationship of various items to a single base item within a single period (running vertically down the column), it is widely referred to as vertical analysis.
Vertical analysis is highly useful for comparing financial statements of different-sized companies (inter-firm comparison) or comparing the performance of a single company over different periods (intra-firm comparison) because it scales all figures to percentages, eliminating the bias of absolute values.