Step 1: Recall the historical context.
Before Maxwell, electricity, magnetism, and optics were treated as separate subjects. The question asks which single discovery tied optics to electromagnetism.
Step 2: Maxwell's key result.
Maxwell showed that oscillating electric and magnetic fields can travel through space as a wave moving at speed $c=\dfrac{1}{\sqrt{\mu_0\epsilon_0}}$.
Step 3: Compare with measured light speed.
Plugging in $\mu_0$ and $\epsilon_0$ gives about $3\times10^8$ m/s, exactly the measured speed of light. This match strongly suggested light itself is such a wave.
Step 4: Test option (2).
Light does not travel at the speed of sound (sound is about $340$ m/s), so this is wrong.
Step 5: Test options (3) and (4).
Light is not a stream of electrons, and ordinary light beams are not bent by static electric or magnetic fields, so both are wrong.
Step 6: Conclude.
The unification rests on recognising that light is an electromagnetic wave, which is option (1).
\[ \boxed{\text{Light is an electromagnetic wave}} \]