Correct Answer - B
We have
`cos.pi/7+cos.(2pi)/7+cos.(3pi)/7+cos.(4pi)/7+cos.(5pi)/7+cos.(6pi)/7+cos.(7pi)/7`
`=(cos.pi/7+cos.(6pi)/7)+(cos.(2pi)/7+cos.(5pi)/7)+(cos.(3pi)/7+cos.(4pi)/7)+cospi`
`=(cos.pi/7-cos.pi/7)+(cos.(2pi)/7-cos.(2pi)/7)+(cos.(3pi)/7-cos.(3pi)/7)+cospi`
`=cospi=-1`