A wooden block is floating in a liquid, $$40\ \%\ $$ of its volume is inside the liquid when the vessel is stationary. Percentage of volume immersed when the vessel moves upwards with an acceleration $$a=5\ m.s^{-2}$$ is

For the block to float, the buoyant force must balance its effective weight.

When the vessel is stationary,  

$$\rho_l V_{\text{immersed}}g=\rho_b Vg$$

Given that $$40\%$$ of the volume is immersed, 

$$\frac{V_{\text{immersed}}}{V}=0.4$$

When the vessel accelerates upward, the effective acceleration becomes $$g+a$$. Hence,

$$\rho_l V_{\text{immersed}}(g+a)=\rho_b V(g+a)$$

The factor $$(g+a)$$ cancels from both sides, giving

$$\frac{V_{\text{immersed}}}{V}=\frac{\rho_b}{\rho_l}=0.4$$

Thus, the percentage of volume immersed remains unchanged which is $${40\%}$$

Hence, the correct option is B.