Step 1: Separate "where things are" from "how gravity behaves".
Geodesy uses two related but distinct kinds of models. One kind fixes coordinates, an origin, axes orientation, and an ellipsoid so that every location on Earth gets a well defined position, this is a reference frame or datum. The other kind describes the Earth's gravity field or geoid shape, used for converting between height types, this is a gravity/geoid model, a different thing entirely.
Step 2: Sort the four listed items into these two categories.
WGS84 fixes an ellipsoid, an origin at the Earth's centre of mass, and axis directions, it is a coordinate reference frame used worldwide by satellite navigation. ITRF14 is the internationally adopted, continuously refined terrestrial reference frame, again a coordinate reference frame, arguably the most precise one in routine geodetic use. NAD83 similarly fixes a datum, based on GRS80, specifically for North America, again a coordinate reference frame.
Step 3: Place EGM96 in the correct category.
EGM96 belongs to the other category entirely, it is a global geopotential (gravity field) model expressed as spherical harmonic coefficients up to a certain degree and order, from which geoid heights $N$ can be computed at any location. It is used together with a reference frame, for example to turn a GPS ellipsoidal height $h$ referenced to WGS84 into an orthometric height $H = h - N$, but it does not itself supply coordinates or define a datum origin.
Step 4: Conclude.
Since three of the four choices genuinely fix coordinates and one of them, EGM96, only describes the gravity field/geoid and is used alongside a datum rather than being one, EGM96 is correctly identified as not a geodetic reference frame.
\[ \boxed{\text{EGM96}} \]
