Explanation:

- The population of adults is 80% of the town's total population.

- The ratio of men to women among adults is 2:3.

- If the number of adult men is 1.2 lakh, we can calculate the total adult population.

- Let the number of adult men be $2x$ and women be $3x$, so $2x=1.2$ lakh.

- Solving for $x$, we get $x=0.6$.

- Total adult population is $2x+3x=5x=3$ lakh.

- As adults constitute 80% of the total population, the total is $3$ lakh / 0.8 = 3.75 lakh.

Therefore, the correct option is:

- Option 4: 3.75 lakh