A gradation curve is a plot of
\[
\boxed{
\text{Percentage finer} \;\text{vs.}\; \text{Particle size}.
}
\]
It is used to classify soils as well graded or poorly graded.
Step 1: Recall the particle size distribution graph.
The distribution of soil particles of different sizes is represented by a graph obtained from sieve and hydrometer analyses.
Step 2: Identify the graph.
This graph is called the
\[
\boxed{\text{Gradation curve}.}
\]
Hence,
\[
\boxed{(A)}
\]
is the correct answer.