As here we have 7 terms so we can assume the terms of the A.P. as : a - 3d , a - 2d , a - d , a , a + d , a + 2d and a + 3d
Thus sum of 7 terms of the A.P, = (a - 3d) + (a - 2d) + (a - d) + (a) + (a + d) + (a + 2d) + (a - 3d)
= 7a
= 7(8) = 56 [ As fourth term of A.P. = a = 8 is given in the question ]
Hence C) should be the correct answer.