Function of carapace
Preface:


Any question about adaptation on macro-level has very little meaning and no precise answer.
There are two types of larvae in invertebrates: primary ("original") and secondary (evolved from post-larval stages). Not digging into the details, nauplius (together with metanauplius) is the only type of primary larvae in Crustacea and it never has a carapace.
"Majority" is by far too strong term, even if we are talking about all larval types. If we just take the largest crustacean orders with species numbers > 1000, together comprising 92% of crustacean species (species numbers are from Ahyong et al, 2011, they seem to be largely correct):


Decapoda - 14.7 k species - rarely have nauplius, marine forms do have planktonic carapaced larvae (principally - zoea)
Isopoda - 10.7 k - no carapace
Amphipoda - 9.9 k - no carapace
Podocopida - 6.2 k - no carapace
Harpacticoida - 6.0 k - no carapace
Cyclopoida - 4.2 k - no carapace
Calanoida - 3.1 k - no carapace
Siphonostomatoida - 2.5 k - no carapace
Cumacea - 1.5 k - have carapace, but development is direct
Mysida - 1.2 k - have carapace, but development is direct
Tanaidacea - 1.1 k - have carapace, but development is direct



So, "majority" could be applied to Decapoda only.

Fabula:

If we substitute "adaptation" with "main biological function" (as you have done) or more correctly "main physiological role":

I would prefer the (a), because carapace does function as a protective structure in those decapod larvae which don't rely on behavioral anti-predatory defenses: see Morgan: "Fishes quickly learned to avoid spined prey, which ... may also increase the rate of evolution of the character". Non-carapaced larvae rely principally on behavior only.

The only contribution of carapace to planktonic life-style in zoea is participation of its spines [when they are pronounced] in parachuting (Chia et al).

[The above should be largely true also for the cypris larva of the cirripeadians, which are enclosed in a non-spined bivalve carapace.]

From the morphological point of view, [at least in adult forms of decapods] carapace creates a chamber for gills, so a morphologist would argue that this is the "main function" of carapace.