Explanation:

- Average without new student: The average marks of the original three students is $x$, so their total marks is $3x$.

- Average with new student: The new student increases the average by 2, making it $x+2$. Since there are now four students, their total marks is $4(x+2)$.

- Equation setup: Equating the two total marks expressions, we have:

$$3x+\text{(marks of new student)}=4(x+2)$$

- Simplify equation:

$$3x+\text{(marks of new student)}=4x+8$$

$$\text{Marks of new student}=4x+8-3x$$

$$\text{Marks of new student}=x+8$$

- Correct Option: The marks secured by the new student is $x+8$.

X + 8 is the correct answer.