The thin metallic strip of vernier calipers moves downward from top...

We are measuring the depth from metallic strip of vernier calipers.

Given,

The least count of vernier calipers& $LS = 0 . 1mm = 0 . 01cm$

Main scale reading~$MSR = 6 . 4cm$

Vernier scale reading~$VSR = 4$

So the reading of the depth,$D = MSR + ( VSR \times LC ) = 6 . 4 + ( 4 \times 0 . 01 ) = 6 . 44cm$

Because, there is no zero error, so depth will be same as above.