Answer
(a) simple harmonic,
(b) $2\ m$,
(c) $1\ sec$,
(d) $1\ Hz$.
Work Step by Step
(a) Given $d=6+2cos(2\pi t)$, we can describe the motion of the object as simple harmonic,
(b) The maximum displacement from its resting position can be found as $2\ m$,
(c) We can find the period as $p=\frac{2\pi}{2\pi}=1\ sec$,
(d) The frequency is the reciprocal of the period $f=\frac{1}{p}=1\ Hz$.