You have confused the effects of a and b in $\displaystyle{y}={a}{\cos{{b}}}{x}\usepackage{cmbright}\Huge\$ . The amplitude of this graph is 3, while the amplitude of $\displaystyle{y}=-\frac{{1}}{{3}}{\cos{{3}}}{x}\usepackage{cmbright}\Huge\$ is $\displaystyle\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$ . The graph shows half of a cycle on an interval of length $\displaystyle{3}\pi\usepackage{cmbright}\Huge\$ , so the period of the graph is $\displaystyle{6}\pi\usepackage{cmbright}\Huge\$ . The period of $\displaystyle{y}=-\frac{{1}}{{3}}{\cos{{3}}}{x}\usepackage{cmbright}\Huge\$ is $\displaystyle\frac{{{2}\pi}}{{3}}\usepackage{cmbright}\Huge\$ . The correct answer is the graph passing through

$\displaystyle{\left({0},-\frac{{1}}{{3}}\right)}\usepackage{cmbright}\Huge\$ .