Step 1: Identify the intended meaning from context.
The sentence talks about children in the rains who “run the risk of getting lost.” This risk spikes when a child is without an adult/caretaker. The legal/administrative phrase for this is unaccompanied (e.g., “unaccompanied minors”).
Step 2: Eliminate close distractors by sense and collocation.
(A) Lonely = feeling of sadness due to being alone; an emotion, not a supervision status. ✗
(B) Solitary = existing alone; stylistically odd with “children” (plural) and still describes state, not supervision. ✗
(C) Single = one in number / unmarried; unfit for a group of children. ✗
(E) Unguarded = without guard/protection; collocation with children is uncommon and focuses on security, not presence of an accompanying adult. ✗
(D) Unaccompanied precisely encodes “no responsible companion present.” ✓
Step 3: Conclude.
“Unaccompanied children in rains run the risk of getting lost.”
\[
\boxed{\text{Option (D)}}
\]