We can see here that the purpose of the two metaphors Kennedy uses in the phrase: "If a beachhead of cooperation may push back the jungle of suspicion..." (paragraph 20) is: B.) Kennedy uses the metaphors to conclude his argument that the U.S. and its adversaries must cooperate and reject their suspicions of each other.
The metaphor of "beachhead" refers to establishing a starting point or foothold for cooperation, similar to the strategic military concept of securing a beachhead before launching a broader campaign. The "jungle of suspicion" represents the barriers of mistrust and skepticism between nations.
Kennedy employs these metaphors to underscore the necessity of initiating cooperation as a means to overcome mutual distrust and build a foundation for better relations between the United States and its adversaries.