Step 1: Basic fact:
The emf in the secondary coil of a transformer is $\varepsilon_s=-N_s\,d\Phi/dt$. This needs a flux that changes with time.
Step 2: Answer to (a):
A dc source sends a constant current. Constant current means constant flux, so $d\Phi/dt=0$ and $\varepsilon_s=0$ (apart from a brief spike when the switch is closed). So a transformer can neither step up nor step down dc.
Step 3: Answer to (b):
The ratio $V_s/V_p=N_s/N_p$ decides the action. In a step-up unit, $N_s>N_p$.
If we swap the roles, the long coil becomes the primary and the short coil becomes the secondary. The new ratio $N_s/N_p$ is less than 1, so the output voltage drops. So yes, the same device can be used as a step-down transformer.
Step 4: Answer to (c):
Compare powers. In an ideal case $V_sI_s=V_pI_p$. If $V_s=10V_p$, then $I_s=I_p/10$. The power is the same on both sides.
In an actual transformer, some energy is lost as heat in the windings and the core, so $P_{out}<P_{in}$. Energy is never created, so there is no violation.
Final Answer:
(a) No. (b) Yes. (c) No, since a larger voltage comes with a smaller current and $P_{out}\le P_{in}$.