Correct Answer - B
In SHM, `x=a sin omega t `
When `x=A//2,` then
`(A)/(2)=Asin omegat`
or `sin omegat =(1)/(2) or omega t=(pi)/(6) or (5pi)/(6)`
`:.` Phase of 1st particle, `phi_(1)=(pi)/(6)rad`
Phase of 2nd particle, `phi_(2)=(5pi)/(6)rad`
Phase difference,
`=phi_(2)-phi_(1)=(5pi)/(6)-(pi)/(6)=(2pi)/(3)rad`