For the hydrides of nitrogen family going down the group:
A. Stability decreases
B. Reducing activity increases
C. Bond angle HMH decreases
D. All the above variations followed

Hint: We need to study the property trends for the hydrides of nitrogen families. Hydrides are hydrogen compounds which combine with less electronegative elements by loose covalent bonds. For example, water is called an oxide of hydrogen. The Nitrogen family belongs to group 15 of the periodic table whose elements are $N$, $P$, $As$,$Sb$, $Bi$ . We shall study the property trends of the hydrides of these elements down the group.

Complete step by step answer:
We must remember that the hydrides of group 15 elements are $N{H_3}$, $P{H_3}$, $As{H_3}$, $Sb{H_3}$,$Bi{H_3}$. We now analyse the properties of these hydrides as we go down the group.
Stability: We know that elements at the top of the group have a smaller atomic size then the ones at the atom. In other words, atomic size increases as we go down the group. Increase in atomic size means valence electrons which bond with the hydrogens to form hydrides are far from the nucleus hence experience less force of attraction by the nucleus. Hence their stability decreases down the group.
$N{H_3} > P{H_3} > As{H_3} > Sb{H_3} > Bi{H_3}$
Reducing activity: Reducing activity of hydrides is the property to readily donate its hydrogen atom. Since atomic size increases as we go down the group, the bond strength decreases hence the stability decreases and the reducing activity naturally increases.
Bond angle of HMH: Since atomic size increases as we go down the group, the bond-pair bond-pair repulsion decreases with a pyramidal shape with one lone pair of electrons. Hence the bond angle between Hydrogen-Metal-Hydrogen decreases.

So, the correct answer is Option D.

Note: We must be noted that another aspect for studying the stability of the hydrides of group 15 is the overlapping of the central metal atom with the hydrogen. Smaller is the size of the central metal atom stronger will be the overlapping taking place with the hydrogen atom. Nitrogen is the smallest element of group 15 so there occurs effective overlapping between the nitrogen and hydrogen atom.