Can the change in internal energy of an ideal gas be non-zero during an isothermal process?

In which of the processes does the internal energy of the system remain constant?

When an ideal gas is compressed isothermally,its pressure:

During which thermodynamic process does $T$ remain constant, while the other two variables $P$ and $V$ change?

$A$ cylinder fitted with a piston contains $0.2 \, moles$ of air at a temperature of $27^{\circ}C$. The piston is pushed so slowly that the air within the cylinder remains in thermal equilibrium with the surroundings. Find the approximate work done by the system if the final volume is twice the initial volume (in $J$).

Two ideal gases $A$ and $B$ of the same number of moles expand at constant temperatures $T_1$ and $T_2$ respectively such that the pressure of gas $A$ decreases by $50 \%$ and the pressure of gas $B$ decreases by $75 \%$. If the work done by both the gases is same, then $T_1: T_2$ is: