(a) is right.

(b) is less idiomatic; it looks like a past in the past.

(c) should be C’est bon de finir son travail.

(d) ditto: should be C’est bon d’avoir fini son travail.

I agree with @jiliagre: the second feels like your vacations were before another event to which “c’etait” would be attached. In terms of style, “je me suis bien detendu pendant les vacances” is more direct and about equivalent.