(a) $3\pi d^2$
Diameter = $2d$, so radius $r = d$.
TSA of solid hemisphere $= 2\pi r^2 + \pi r^2 = 3\pi r^2 = 3\pi d^2$.
TSA of a solid hemisphere = curved surface area ($2\pi r^2$) + circular base area ($\pi r^2$) = $3\pi r^2$. Here $r = d$, so the answer is $3\pi d^2$. Don't forget the flat circular base — a common mistake.