Short answer: "That" is used to introduce essential clauses or information, while the pronoun "which" is used to add non-essential or non-defining clauses. Also, these non-essential clauses are set off with commas.
Explanation:
First of all, the words "that" and "which" are both relative pronouns; this means, both introduce clauses that describe a noun previously mentioned. However, the pronoun "that" should be used only to introduce essential or defining clauses (information that cannot be omitted); for example "She is the woman that I love". On the other hand, the word "which" introduces information that is not essential or non-defining clauses; for example "The house, which is located near the sea, is stunning". Moreover, clauses introduced by which are set off in commas to show they are not essential.
B: negotiations for inspection and control of weapons
Explanation:
President John Fitzgerald Kennedy from the quote above is talking about negotiations to inspect and control weapons that's why he called for formulation of serious and workable plans and proposals among countries to control and inspect weapons of destruction.
