Explanation:

Let's go through the problem step by step:

- Let the present age of the elder son be $x$.

- Then, the father's present age will be $3x$ (as the father is three times the age of the elder son).

- For the younger son, let the age be $y$. The elder child is 6 years older than the younger one, so $x=y+6$.

- Four years hence, the father's age will be $3x+4$, and it is given that this age will be four times the age of the younger son at that time, i.e., $4(y+4)$.

- Equating the above equations, we get: $3x+4=4(y+4)$.

Now, substituting $x=y+6$ into the equation:

- $3(y+6)+4=4(y+4)$

- $3y+18+4=4y+16$

- Simplifying, we get: $3y+22=4y+16$

- Solving for $y$, we find: $y=6$.

Now, substituting back to find the father's age:

- $x=y+6=12$

- Father's age is $3x=36$.

Thus, the correct answer is:

- Option:1, 36 years