Answer
$2.87^\circ$
Work Step by Step
The two buildings here and the space between them are representing a single-slit situation where the distance between them is the width of the slit.
The electromagnetic wave travels at the speed of light, and since we know its frequency, we can easily find its length.
$$\lambda=\dfrac{c}{f}\tag 1$$
The author here asks about the angular width of the electromagnetic wave after it emerges from between the buildings, which is the angular width of the central maximum fringe. This width is given by the distance between the first two minima frginges before and after the central maximum fringe.
Hence, $$\theta_{\rm central\;maximum}=2\theta_1\tag 2$$
where $\theta_1$ is given by the dark fringes of
$$a\sin\theta_p=p\lambda$$
where $p=1$,
$$ \sin\theta_1= \dfrac{\lambda}{a}$$
$$ \theta_1=\sin^{-1}\left[ \dfrac{\lambda}{a}\right]$$
Plugging from (1);
$$ \theta_1=\sin^{-1}\left[ \dfrac{c}{af}\right]$$
Plugging the known;
$$ \theta_1=\sin^{-1}\left[ \dfrac{3\times 10^8}{(15)(800\times 10^6)}\right]$$
Plugging into (2);
$$\theta_{\rm central\;maximum}=2 \sin^{-1}\left[ \dfrac{3\times 10^8}{(15)(800\times 10^6)}\right]$$
$$\theta_{\rm central\;maximum}=\color{red}{\bf 2.87}^\circ$$
