Step 1: Fix the classifying rule.
An amine is named by how many carbon groups sit directly on the nitrogen: one carbon means primary, two means secondary, three means tertiary.
Step 2: A quick visual cue.
Primary nitrogen carries $-\text{NH}_2$, secondary carries $-\text{NH}-$ between two carbons, and tertiary carries no N-H at all.
Step 3: Check hexane-1,6-diamine.
Both ends are $-\text{NH}_2$ groups, each nitrogen touching only one carbon, so these are primary centres.
Step 4: Check N,N-dimethylbenzenamine.
The nitrogen bears a phenyl ring plus two methyl groups, that is three carbon groups, making it tertiary.
Step 5: Check N-methylbenzenamine.
Here nitrogen is flanked by a phenyl ring on one side and a methyl on the other, leaving a single N-H. Two carbon groups means this is the secondary amine.
Step 6: Check prop-2-en-1-amine.
The nitrogen sits as a terminal $-\text{NH}_2$ on the allyl chain, so it is primary.
Step 7: Choose the match.
Only N-methylbenzenamine fits the two-carbon secondary pattern, so the answer is option (3).
\[ \boxed{\text{N-methylbenzenamine (secondary amine)}} \]