Step 1: Recall the concept of hematocrit.
The hematocrit is the percentage of blood volume made up of red blood cells (erythrocytes). In a healthy adult, hematocrit is about 40% to 45%, meaning nearly half of the blood's volume is packed with erythrocytes alone.
Step 2: Compare cell counts numerically.
A normal adult has roughly $5 \times 10^6$ erythrocytes per microliter of blood.
By contrast, total white blood cells (lymphocytes, basophils, and neutrophils combined) number only about $4 \times 10^3$ to $1.1 \times 10^4$ per microliter.
The ratio of erythrocytes to all white blood cells combined is roughly $\frac{5 \times 10^6}{10^4} \approx 500$.
Step 3: Rank the four given options by this count.
Erythrocytes: about $5 \times 10^6$ per microliter, highest by far.
Neutrophils: about $2.5$ to $7 \times 10^3$ per microliter, the most common white cell, but still small next to erythrocytes.
Lymphocytes: about $1$ to $4 \times 10^3$ per microliter.
Basophils: less than $100$ per microliter, the rarest of the four.
Step 4: Pick the correct option.
Since erythrocytes outnumber every white cell type by several hundred times, they are the most abundant cell in blood. This corresponds to option (A).
Final Answer:
Erythrocytes are the most abundant cell type in healthy adult human blood.
\[ \boxed{\text{Erythrocytes}} \]