a
(a) Volume of log of wood \(V = \frac{{{\rm{mass}}}}{{{\rm{density}}}} = \frac{{120}}{{600}}=0.2 m^3\)
Let x weight that can be put on the log of wood.
So weight of the body = \((120 + x) \times 10\;N\)
Weight of displaced liquid = \(V\sigma g = 0.2 \times {10^3} \times 10\;N\)
The body will just sink in liquid if the weight of the body will be equal to the weight of displaced liquid.
 \((120 + x) \times 10 = 0.2 \times {10^3} \times 10\)
\( \Rightarrow \;120 + x = 200\) 

\(x = 80 kg\)