Explanation

(d)
Let $4,\ G_{1},\ G_{2},\ G_{3},\frac{1}{4}$ are in GP.
$\therefore\ G_{1} = ar = 4r$
$G_{2} = 4r^{2},\ G_{3} = 4r^{3}$
$G_{4} = 4 \times r^{4} = \frac{1}{4}$
$\Rightarrow \ r = \frac{1}{2}$
$\therefore$ Product of GM $= G_{1} \bullet G_{2} \bullet G_{3}$
$= ar \bullet ar^{2} \bullet ar^{3}$
$= a^{3}r^{6}$
$= 4^{3} \times \left( \frac{1}{2} \right)^{6} = \frac{4^{3}}{4^{3}} = 1$