Japt, 7 bytes

ËÆD
c ö

Test it here

Explanation

Implicit input of array U.

Ë

Map over the array passing each element through a function where D is the current element.

ÆD

Generate an array of length D and fill it with D.

c

Flatten.

ö

Get a random element.

Japt, 7 6 bytes

cÈÇX
ö

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cÈÇX\nö     :Implicit input of array U
c           :Flat map by
 È          :Passing each X through the following function
  ÇX        :  Map the range [0,X) to X
    \n      :Reassign to U
      ö     :Random element

#Japt, 7 bytes

ËÆD
c ö

Test it here

##Explanation Implicit input of array U.

Ë

Map over the array passing each element through a function where D is the current element.

ÆD

Generate an array of length D and fill it with D.

c

Flatten.

ö

Get a random element.

Japt, 7 bytes

ËÆD
c ö

Test it here

Explanation

Implicit input of array U.

Ë

Map over the array passing each element through a function where D is the current element.

ÆD

Generate an array of length D and fill it with D.

c

Flatten.

ö

Get a random element.

#Japt, 7 bytes

ËÆD
c ö

Test it here

##Explanation Implicit input of array U.

Ë

Map over there array passing each element through a function where D is the current element.

ÆD

Generate an array of length D and fill it with D.

c

Flatten.

ö

Get a random element.

#Japt, 7 bytes

ËÆD
c ö

Test it here

##Explanation Implicit input of array U.

Ë

Map over the array passing each element through a function where D is the current element.

ÆD

Generate an array of length D and fill it with D.

c

Flatten.

ö

Get a random element.