(d)
$\frac{\log_{c + b}{a + \log_{c - a}a}}{2\log_{c + b}a.\log_{c - b}a\ }$
$= \frac{\frac{\log a}{\log{(c + b)}} + \frac{\log a}{\log{(c - b)}}}{2\frac{\log a}{\log{(c + b)}}.\frac{\log a}{\log{(c - b)}}}$
$= \frac{\log{a\{\log{(c - b) + \log{(c + b)}}\}}}{2\left( \log a \right)^{2}} = \frac{\log{(c^{2} - b^{2})}}{2\log a}$
$= \frac{\log a^{2}}{\log a^{2}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (\because a^{2} + b^{2} = c^{2})$
$= 1$