Answer
$a.$ True;
$b.$ Not true;
$c.$ True.
Work Step by Step
$a.$ The graph of the linear function is a straight line. Any secant or a tangent of a straight line is that very same straight line, so this statement is true.
$b.$ This doesn't have to be true. One example is the linear function where the slopes are equal. Another conterexample is in the next part.
$c.$ The slope of the secant line is given by
$$m_{sec}=\frac{f(x+h)-f(x)}{h}=\frac{(x+h)^2-x^2}{h}=\frac{x^2+h^2+2xh-x^2}{h}=\frac{h^2+2xh}{h}=h+2x.$$
The slope of the tangent at point $P$ is given by
$$m_{tan}=\lim_{h\to0}m_{sec}=\lim_{h\to0}(h+2x)=0+2x=2x.$$
Since $h>0$ we see that $2x+h>2x$ which means that $m_{sec}>m_{tan}$ so the statement is correct.
