For $n$ records, blocks required = $\frac{n}{3}$
For $n$ indices in the index file, blocks required = $\frac{n}{10}$
Total = $\frac{n}{3}+\frac{n}{10}$ = $\frac{10n}{30}+\frac{3n}{30}$ = $\frac{13n}{30}$
Option A
Bonus:-
In case of sparse indexing:
For $n$ records, blocks required = $\frac{n}{3}$
For $\frac{n}{3}$ indices in the index file, blockes required = $\frac{n}{30}$
Total = $\frac{n}{3}+\frac{n}{30}$ = $\frac{10n}{30}+\frac{n}{30}$ = $\frac{11n}{30}$
For dense indexing, we have a pointer (index in the index file) for each record.
For sparse indexing, we have a pointer for each block. (More specifically for every first record of the block)