(C) $\dfrac{10}{0.2}$
Since $\dfrac{10}{0.2} = 50$, which is greater than 1, it cannot be the probability of an event (probability must satisfy $0 \leq P(E) \leq 1$).
Source: Chapter 14, Section 14.1
The key rule is $0 \leq P(E) \leq 1$. Check each option: (A) 39/100 = 0.39 ✓, (B) 0.001/20 = 0.00005 ✓, (D) 10% = 0.1 ✓ — all lie between 0 and 1. Only (C) equals 50, which exceeds 1, so it is impossible as a probability.