Indexed/grouped allocation (of disk blocks to a file) is useful because:
(A) It supports both sequential and direct access.
(B) The entire data block is available for data (no pointers embedded in it).
(C) It does not require a lot of space for keeping pointers.
(D) It has no external fragmentation.

Explanation

In indexed allocation, every disk block address for a file is kept in a separate index block, so (A) holds — you can jump straight to any block via the index (direct access) or walk the index in order (sequential access). Because the pointers live only in the index block, each data block is used entirely for file data, so (B) holds. Blocks can be allocated from anywhere on disk and linked through the index rather than needing one contiguous run, so there is no external fragmentation (D). (C) is false and is a genuine drawback of indexed allocation: it needs a whole extra block (or more, for large files) just to store pointers, which is overhead that linked allocation does not have for small files. Hence the correct set is A, B and D only, option (iv).