Given data:
The height of block A is h₁=10 cm.
The heigh of of block B is h₂=5 cm.
The block A is sinked by x₁=7 cm.
The fraction of block A in the water will be,

The height submerged in the water by block will be,

Thus, the block B is 3.5 cm in the water.
Option B is correct.