The function \( f(x) = 5x + 5 \) is a linear function.
To determine the nature of the function, we can look at the coefficient of \( x \). In this case, the coefficient is \( 5 \), which is positive. This means that as \( x \) increases, \( f(x) \) also increases.
Therefore, the true statement is:
a. The function is an increasing function.