Correct Answer - Option 3 : Bolzano-weierstrass theorem
Concept:
Every bounded infinite set of real numbers has at least one limit point or cluster point. The theorem is known as Bolzano-Weierstrass Theorem
Some other important theorems by Bolzano-Weierstrass are:
1. Every bounded sequence has a convergent subsequence.
2. A sequence {an} converges if and only if it is unbounded and has exactly one subsequential limit
Cauchy's theorem
If f(z) is analytic everywhere within a simply-connected region then:
\(\oint_c f(z)\ dz = 0\)
for every simple closed path C lying in the region