b) This is a function as for each value of speed, there is one value of time taken in hours. In fact, because of the data given, this function can be modeled by the equation: \(\displaystyle{f{{\left({x}\right)}}}=\frac{{100}}{{x}}\)

c) This is a function because the of the given statement, the output, will always be equal to the input.

d) This is a function because for each individual year, there is one unique value of the population of the United States.