What defines a frequency in electronics?
Any signal (of any shape) can be the lowest frequency single sinewave imaginable or, a massive concoction of billions of individual sinewaves. What you have drawn (a, b or c) is reproducable (and analysable) as just this - a bunch of sinewaves. If you can draw limits around it where it might repeat then it's realizable mathematically as a series of individual sinewaves (but possible an infinite number in the case of a square wave).

The blue wave will certainly have a 1.5Hz and possibly 3Hz component that is bigger than say a 2Hz or 4Hz component but, because there is a gradual slope upwards it is impossible to say what its lower frequency content will be. It's always easiest to see the lowest component perform one full cycle before being definite about it. For instance the 1.5Hz components may actually dissapear if the full picture was known.

This happens when two sinewaves are multiplied with each other - one component may possibly disappear completely.