While it is true that you can use the angles to determine if two triangles are similar, there is another way to find this out. You need to look at the relationship between the corresponding sides.

The correct answer is $\displaystyle\Delta{A}{B}{C}\usepackage{cmbright}\Huge\$ ~ $\displaystyle\Delta{D}{E}{F}\usepackage{cmbright}\Huge\$ because: $\displaystyle\frac{{4}}{{6}}=\frac{{5}}{{7.5}}=\frac{{6}}{{9}}\usepackage{cmbright}\Huge\$ .