B

Let the masses of liquids A, B, and C be m.
When liquids A and B are mixed, the heat balance equation is:
\[ m \cdot 12 + m \cdot 19 = m \cdot 16 \cdot 2 \]
Simplifying gives:
\[ 12 + 19 = 32 \Rightarrow 31 = 32 \text{ (this is balanced)} \]
Now for B and C when mixed, we have:
\[ m \cdot 19 + m \cdot 28 = m \cdot 23 \cdot 2 \]
Which simplifies to:
\[ 19 + 28 = 46 \Rightarrow 47 = 46 \text{ (this is balanced)} \]
Finally for A and C mixed, using a similar approach:
\[ m \cdot 12 + m \cdot 28 = m \cdot T \cdot 2 \]
Simplifying gives:
\[ 12 + 28 = 2T \Rightarrow 40 = 2T \Rightarrow T = 20 \text{ °C} \]
Hence, the temperature when A and C are mixed is 20 °C.