Answer a. Independence means that the probability of an event (for example, head or tail in a coin flip) does not affect subsequent coin flips and is not affected by previous coin flips. The number of trials is unlimited.
Dependence means that the probability of an event affects the probability of a subsequent event. For example, drawing a particular suit from a pack of cards without replacing it into the pack changes the probability of drawing the next card, because there will be one fewer card and one less suit remaining.