The proton (which consists of two up quarks and a down quark) has a strangeness of zero. The $\pi^-$ , which consists of a $\bar{u}$ quark and a $d$ quark, also has a strangeness of $0$ . The $\Lambda$ , which consists of an up quark, a down quark, and a strange quark, has a strangeness of $-1$ . %(See Table 14.6 in Modern Physics)
Therefore, strangeness is not conservation in this reaction. Nonconservation of strangeness implies that the reaction proceeds via the weak interaction. %(See pages 523 and 524 of Modern Physics).
Therefore, answer (E) is correct.

Solution to 1992 Problem 91