Correct Answer - c
Since the proton is entering the magnetic field at some angle
other than `90^@` , its path is helix.
Corresponding velocity of the proton along X-axis,
`v_x=vcos 60^@=2xx10^6xx1/2=10^6ms^-1`
Due to velocity component `v_x`, the radius of the helix is
described and is given by the relation
`r=(mv_x)/(qB) =(1.6xx10^-27xx10^6)/(1.6xx10^-19xx0.10)=0.1m`
Now, `T=(2pir)/(v_x) =(2pixx0.1)/(10^6)=2pixx10^-7s`