Answer:
Option 2nd is correct

Step-by-step explanation:
The nth term for the arithmetic equation is give by:
....[A]
where,
is the first term
d is the common difference and
n is the number of term.
As per the statement:
The fourth term in an arithmetic sequence is 18 and the seventh term is 42
⇒
and 
then by above definition we have;
.....[1]
.....[2]
Subtract equation [1] from [2] we have;
3d = 24
Divide both sides by 3 we have;
d = 8
Substitute in [1] we have;

Subtract 24 from both sides we have;

We have to find the equation for the nth term of this sequence
Substitute the given values in [A] we have;

⇒
⇒
Therefore, an equation for the nth term of this sequence is, 