$9 \text { M.S.D }=10 \text { V.S.D }$

$\text { 1 V.S.D }=0.9 \text { M.S.D }=0.9\,mm$

$\text { L.C of vernier caliper }=1-0.9=0.1\,mm =0.01\,cm$

$\text { zero error }=-(10-4) \times 0.1\,mm =-0.6\,mm$

$\text { Reading }=\text { M.S.R }+\text { V.S.R }-\text { Zero error }$

$=3\,cm +6 \times 0.01-[-0.06]$

$=3+0.06+0.06$

$=3.12\,cm$

Nearest given answer in the options is $3.10$