What is X in the sequence 1, 3, 6, 11, 18, X, 42?

The correct option is 29.

To find the missing value $X$ in the sequence, let us analyze the differences between consecutive terms:

$3-1=2$

$6-3=3$

$11-6=5$

$18-11=7$

Notice that the differences between consecutive terms are $2$, $3$, $5$, and $7$, which are consecutive prime numbers.

Following this pattern, the next prime number after $7$ is $11$.

Therefore, we can calculate $X$ by adding $11$ to the preceding term $18$:

$X=18+11=29$

To verify, the prime number following $11$ is $13$. Adding $13$ to $29$ gives:

$29+13=42$

This matches the final term in the given sequence. Thus, $X=29$.