The odds in favor of an event \( E \) occurring are given as \( a : b \), meaning that for every \( a \) favorable outcomes, there are \( b \) unfavorable outcomes. The probability of event \( E \) is given by:
\[
P(E) = \frac{\text{favorable outcomes}}{\text{total outcomes}} = \frac{a}{a + b}.
\]
Thus, the correct answer is option (B) \( \frac{a}{a+b} \).