(C) The sum of $n$ terms of an $AP$ is given by the formula: $S_{n} = \frac{n}{2}(a + a_{n})$.Given $a = 1$,$a_{n} = 20$,and $S_{n} = 399$.Substitutin

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(C) The sum of $n$ terms of an $AP$ is given by the formula: $S_{n} = \frac{n}{2}(a + a_{n})$.Given $a = 1$,$a_{n} = 20$,and $S_{n} = 399$.Substituting these values into the formula:$399 = \frac{n}{2}(1 + 20)$$399 = \frac{n}{2}(21)$$399 = 10.5n$$n = \frac{399}{10.5}$$n = 38$.Thus,the value of $n$ is $38$.