This is the direct definition of a dense index:
$$\boxed{\text{Dense index}=\text{index entry for every search-key value}}$$
Suppose the data file contains $:10,\ 20,\ 30,\ 40,\ 50$
A dense index contains:
$$\begin{array}{|c|c|} \hline \text{Search Key} & \text{Index Entry}\\ \hline 10 & \text{Pointer}\\ \hline 20 & \text{Pointer}\\ \hline 30 & \text{Pointer}\\ \hline 40 & \text{Pointer}\\ \hline 50 & \text{Pointer}\\ \hline \end{array}$$
All search-key values are represented.
By contrast, a sparse index might contain only:
$$\begin{array}{|c|c|} \hline \text{Search Key} & \text{Index Entry}\\ \hline 10 & \text{Block 1}\\ \hline 30 & \text{Block 2}\\ \hline 50 & \text{Block 3}\\ \hline \end{array}$$
Hence,
Answer : A
