Here we can use the formula

$$\begin{gathered} \text{Average (A)=}\frac{\mathrm{Sum} \mathrm{of} \mathrm{observations} \left(\mathrm{S}\right)}{\mathrm{No}. \mathrm{of} \mathrm{observations} \left(\mathrm{N}\right)} \\ \text{⇒ S=A×N} \end{gathered}$$

According to the question,

Sum of marks scored in 5 subjects (A=40, N=5)

⇒S=40 X 5 =200

Later, adding 6th subject , average becomes 46. (so A=46, N=6)

So, sum of score of 6 th subject

⇒S=46 X 6 =276

Now,

Student score in 6 th subject=(sum of score of 5 subjects)-(Sum of score of 6 subjects)

= 276-200

= 76

Hence, Option B is correct.