Explanation

(d)
When ammonia ($NH_{3}$) reacts with formaldehyde ($HCHO$), hexamethylenetetramine which is also known as urotropine, is formed. Urotropine is used as a medicine to treat urinary infections.
$6HCHO\ \ \ \ \ \ + \ \ \ \ \ \ \ \ 4NH_{3} \rightarrow \ \left( CH_{2} \right)_{6}N_{4} + 6H_{2}O$
formaldehyde ammonia urotropine