Q.In a discrete probability distribution the sum of all the probabilities is always equal to :

For a discrete random variable, the probability mass function (p.m.f.) $p(x_i)=P(X=x_i)$ gives the probability of each individual value, subject to $p_i\geq0$ and $\sum p_i=1$. For a continuous random variable, the probability density function (p.d.f.) $f(x)$ plays the analogous role, subject to $f(x)\geq0$ and $\int_{-\infty}^{\infty}f(x)\,dx=1$ …