$(a)$ Magnetic field lines show the direction (at every point) along which a small magnetised needle aligns (at the point). Do the magnetic field lines also represent the lines of force on a moving charged particle at every point?
$(b)$ Magnetic field lines can be entirely confined within the core of a toroid,but not within a straight solenoid. Why?
$(c)$ If magnetic monopoles existed,how would the Gauss's law of magnetism be modified?
$(d)$ Does a bar magnet exert a torque on itself due to its own field? Does one element of a current-carrying wire exert a force on another element of the same wire?
$(e)$ Magnetic field arises due to charges in motion. Can a system have magnetic moments even though its net charge is zero?

(N/A) No. The magnetic force is always normal to $B$ (remember magnetic force $= q(v \times B)$). It is misleading to call magnetic field lines as lines of force.
$(b)$ If field lines were entirely confined between two ends of a straight solenoid,the flux through the cross-section at each end would be non-zero. But the flux of field $B$ through any closed surface must always be zero. For a toroid,this difficulty is absent because it has no 'ends'.
$(c)$ Gauss's law of magnetism states that the flux of $B$ through any closed surface is always zero,$\int_{S} B \cdot dS = 0$. If monopoles existed,the right-hand side would be equal to the monopole (magnetic charge) $q_m$ enclosed by $S$. Analogous to Gauss's law of electrostatics,$\int_{S} B \cdot dS = \mu_0 q_m$,where $q_m$ is the magnetic charge enclosed by $S$.
$(d)$ No. There is no force or torque on an element due to the field produced by that element itself. However,there is a force (or torque) on one element of a wire due to another element of the same wire (though for a straight wire,this net force is zero).
$(e)$ Yes. The net charge in a system may be zero,yet the magnetic moments due to various current loops within the system may not cancel out. We encounter such examples in paramagnetic materials where atoms have a net dipole moment even though their net charge is zero.