S$i$ can be finite or infinite, but since we don't know what the other sets are, we'll treat S$i$ as infinite in order to make the final set S infinite as well.

$\color{red}{\text{Finite Union of Finite Sets is Finite.}}$ i.e. If we have finite number of finite sets, then their union will necessarily be finite.

Union of $S_1,S_2, \dots, S_n$ is infinite set $\text{iff}$ at least one $S_1,S_2, \dots, S_n$ is infinite set.

At least one of the set $S_{i}$ is a finite set. Well, it is not said that$S_{1},S_{2}\ldots S_{n}$ whether they are finite or infinite. It is possible to break down infinite sets into few sets (Some of which can be finite). This seems true, but I'm not able to prove it. Please Give a suitable counterexample here, if you think this is false.

Ex-$: a^{\ast},$ this is infinite set. I can write it as $\{\}\cup \{a^{\ast}\},$ where $\{a^{\ast}\}$ is infinite.

Not more than one of the sets can be finite. This is false.

Ex $: a^{\ast}b^{\ast}\Rightarrow \{ab\} \cup \{\} \cup \{{aa}^{+}{bb}^{+}\}.$

At least one of the sets is Infinite. This must be True. As this is a finite union of sets, one of the sets must be infinite to make the whole thing infinite. True.

Not more than one of the sets $S_i$ can be infinite. This is false.

Ex $: a^{\ast}b^{\ast} = \{a^{p}b^{q}|p=q\}\cup \{a^{m}b^{n}|m\neq n\}$ such that $p,q,m,n \geq 0.$

It's not very difficult to prove option A as False because our task is simple, just to find a case where option A will be false. And indeed we have such a case: If all of $S_{1},S_{2}\ldots S_{n}$ are infinite sets and Infinite Union is Infinite, it becomes false as none of the sets are finite. While in option C, its natural that in order for union to be infinite, at least one of the sets must be an infinite set. Therefore, Ans is Option C.