Answer:
Part a)

Part b)

Explanation:
As per momentum conservation we know that there is no external force on this system so initial and final momentum must be same
So we will have




Part b)
By equation of kinetic energy we have




Your list of choices is a very short list, and doesn't include any
correct explanation.
The mass of an atom is roughly the number of protons AND neutrons
in its nucleus, but the element only depends on how many protons are there.
Different atoms of the same element may have different numbers of
neutrons, so their masses are different. But they're the same element,
because they all have the same number of protons.
The voltage across the other lamp is also 5 V.
In fact, when two resistances are connected in parallel, they are connected to the same points of the circuit. This also means that the potential difference across the two sides of the resistors is the same, therefore the voltage across the two lamps connected in parallel is 5 V for both.