Which choice correctly defines an arithmetic sequence?

Given the sequence $3$, $7$, $11$, $15$, what is the common difference ($d$)?

For the arithmetic sequence with $a_1=2$ and $d=4$, compute $a_{20}$.

Two arithmetic sequences are given by $a_n=6+4(n-1)$ and $b_n=2+6(n-1)$. Find the smallest positive integer $n$ such that $a_n=b_n$.

Find the first $n$ for which the arithmetic sequence with $a_1=50$ and $d=-7$ becomes nonpositive $(a_n\le 0)$.