What is called Gaussian surface ?

Figure shows the electric field lines around three point charges $A, \,B$ and $C$.

$(a)$ Which charges are positive ?

$(b)$ Which charge has the largest magnitude ? Why ?

$(c)$ In which region or regions of the picture could the electric field be zero ? Justify your answer.

$(i)$ Near $A$          $(ii)$ Near $B$          $(iii)$ Near $C$    $(iv)$ Nowhere

A cylinder of radius $R$ and length $L$ is placed in a uniform electric field $E$ parallel to  the cylinder axis. The total flux for the surface of the cylinder is given by-

Find out the surface charge density at the intersection of point $x =3\, m$ plane and $x$ -axis, in the region of uniform line charge of $8\, nC / m$ lying along the $z$ -axis in free space.

A charge $Q\;\mu C$ is placed at the centre of a cube, the flux coming out from any surfaces will be