The correct answer is C. 150 cm.
The effective flange width of a beam is the width of the flange that is effective in resisting bending moment. It is calculated as follows:
$b_e = \frac{b_f}{2} + \frac{d_w}{2}$
where:
- $b_e$ is the effective flange width
- $b_f$ is the flange width
- $d_w$ is the web depth
In this case, we have:
- $b_f = 30$ cm
- $d_w = 50$ cm
Therefore, the effective flange width is:
$b_e = \frac{30}{2} + \frac{50}{2} = 150$ cm
The other options are incorrect because they do not take into account the width of the web.