Concept:
Range of a relation is the set of all possible values of $y$.
Step 1: Form equation
\[
x + 3y = 10
\]
Step 2: Find natural number solutions
\[
x = 10 - 3y
\]
For $x \in \mathbb{N}$:
\[
10 - 3y > 0 \Rightarrow y = 1,2,3
\]
Step 3: Range
\[
\text{Range} = \{1,2,3\}
\]
Final Conclusion:
Option (D)