Which category is A321-271NX in?
The code is for the exact variant for this Airbus:

A321: This is an Airbus A321, the longest variant of the A320 family.

27*: It comes with PW1100 engines (which makes it a neo).

271: The exact engine variant, which can be found in the TCDS (Type Certificate Data Sheet):

PW1133G-JM Geared Turbo Fan jet engines (MOD 161002)


N: Added for all neo variants.

X: This means the aircraft has Cabin Flex, which allows the operator more freedom in choosing emergency exits:

these doors’ configuration flexibility are there to enable all customers’ operational needs without compromising on cabin comfort


(runwaygirlnetwork.com)


The &quot;Cat D&quot; you mentioned is the approach speed categorisation:




Category
Approach Speed




A
&lt; 91 kt


B
91-120 kt


C
121-140 kt


D
141-165 kt


E
166-210 kt




The Airbus A321 can be in approach category C or D, depending on the maximum landing weight:

A321 has Approach Category C to 75.5 tonnes MLW and D beyond that to 77.8 tonnes.

(SKYbrary - Airbus A321)
You can find the maximum landing weights for the A321neo in the Airbus Aircraft Characteristics Document:

All maximum landing weights for the A321neo are above 75.5t, which would make them all Cat D.