C

Step 1: To find the deviation, we can use the formula for the deviation angle \( D \) through a prism: \( D = (n - 1)A \), where \( n \) is the refractive index and \( A \) is the prism angle. Here, \( n = 1.5 \) and \( A = 4^\circ \).
Step 2: Substitute the known values into the formula:
\[ D = (1.5 - 1) \cdot 4 = 0.5 \cdot 4 = 2^\circ \]
Step 3: The question asks for the deviation in the format of \( x \times 10^{-1} \). To express \( 2^\circ \) in this format:
\[ 2 = 20 \times 10^{-1} \]
Hence, the deviation is \( 20 \times 10^{-1} \), which corresponds to option C.