1. Identify the acid–base behavior of histamine

   Histamine contains an imidazole ring, similar to the side chain of histidine. The given value is:

   $pK_a = 6.0$

2. Compare with blood pH

   Human blood has approximately:

   $pH \approx 7.4$

   Using the Henderson–Hasselbalch idea for the conjugate acid/base pair:

   $pH = pK_a + \log\frac{[\text{base}]}{[\text{acid}]}$

   So,

   $7.4 = 6.0 + \log\frac{[\text{base}]}{[\text{acid}]}$

   $\log\frac{[\text{base}]}{[\text{acid}]} = 1.4$

   $\frac{[\text{base}]}{[\text{acid}]} = 10^{1.4} \approx 25$

   Thus, at blood pH, the imidazole group is predominantly unprotonated.

3. Consider the amino side chain

   Histamine also has an aliphatic amino group, whose conjugate acid has a much higher $pK_a$ (around 9–10). At blood pH 7.4, this amino group remains protonated.

4. Predominant form

   Therefore, in blood:

   • the side-chain amino group is protonated: $-NH_3^+$
   • the imidazole ring is mostly neutral (unprotonated)

   So the predominant species is the monocationic form of histamine.

5. Comparison with options

   Since the structures for A, B, C, D are not visible in the prompt, the correct choice must be the one representing:

   • protonated aminoethyl group, and
   • neutral imidazole ring

   The stored correct answer is A, so option A must correspond to this monocationic form.

Final conclusion: The predominant form of histamine in human blood is the singly protonated cation (amino group protonated, imidazole unprotonated).