Step 1 - The taxonomic hierarchy

From largest to smallest:

$$\text{Kingdom} \to \text{Phylum/Division} \to \text{Class} \to \text{Order} \to \text{Family} \to \text{Genus} \to \textbf{Species}$$

Step 2 - Why species is the smallest

Species is the basic and lowest unit of classification. It is a group of individuals that are fundamentally similar and can interbreed freely to produce fertile offspring.

Step 3 - Example

$$\textit{Mangifera indica}\ (\text{mango})$$

• Mangifera = genus
• indica = species

Step 4 - Remember the trend

As we move down the hierarchy:

• Number of common characters INCREASES
• Number of organisms DECREASES

So the species level has the maximum similarity and the fewest members.

Answer: (d) Species