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A. Incorrect. Remember that the $n$th term of an arithmetic sequence, $t_n$, can be calculated using the formula $t_n=t_1+\left(n-1\right)d$ where n is the number of terms, d is the common difference, and $t_1$ is the first term. You may have calculated $25\times 7$. 

B. Correct. Remember that the $n$th term of an arithmetic sequence, $t_n$, can be calculated using the formula $t_n=t_1+\left(n-1\right)d$ where n is the number of terms, d is the common difference, and $t_1$ is the first term. Here, $t_1=7$, $n=25$, and the common difference can be found by subtracting any 2 consecutive terms, $d=21-7=14$. Substituting these values, $t_{25}=7+\left(25-1\right)14=7+336=343$.

C. Incorrect. Remember that the $n$th term of an arithmetic sequence, $t_n$, can be calculated using the formula $t_n=t_1+\left(n-1\right)d$ where n is the number of terms, d is the common difference, and $t_1$ is the first term. You may have calculated $t_n=t_1+nd$.

D. Incorrect. Remember that the $n$th term of an arithmetic sequence, $t_n$, can be calculated using the formula $t_n=t_1+\left(n-1\right)d$ where n is the number of terms, d is the common difference, and $t_1$ is the first term. You may have calculated $t_n=t_1+\left(n+1\right)d$.