Answer: the answer is indeed soft.
Explanation:
From all the options given, the best one is soft. Even though "diffused" originally means scattered or disperse, especially when it refers to light or brightness, the context helps us get to the conclusion that there is a certain softness about the way the light is spread around the room. As we can see below:
<em>In the heat of the glowing July afternoon, the room, with blinds carefully closed, was full of a great calm. From the three windows, through the cracks of the old wooden shutters, came only a </em><em>few scattered sunbeams</em><em> which, in the midst of the obscurity, made a </em><em>soft brightness</em><em> that bathed surrounding objects in a diffused and </em><em>tender light.</em><em> It was cool here in comparison with the overpowering heat that was felt outside, under the fierce rays of the sun that blazed upon the front of the house.</em>
