Three of the following four are alike in a certain way and hence form a group. Which is the one that does not belong to that group?

Here Joystick is the word doesn't belong with the others.

Because Except Joystick all others are Output Devices whereas Joystick is Input Device.

VDU - Visual Display Unit - Monitor to display

Speakers - Output sound

Plotter - used to produce single or multiple colored images.

In all other pairs, second is the tool used by the first. 

Here simple coding language has been used. Each letter has been assigned a numeric code and the respective code of letters are in the same order as the letters. 

Letter      B  E  L  M  O  P  R

Code         8  7  3  5  4  2  9

 BORE -> 8497 -- correct

 MOEP -> 5472 But given 5972 --- Incorrect

 LBOR -> 3849  -- correct

OMEP -> 4572  -- correct

In all other groups, the third and second letters are consecutive and first letter is three steps ahead of the second.

In all other pairs, second is of higher intensity than the first.

In all other pairs , second requires the first to function.

Except 35, all others are obtained by multiplying 5 with a composite Number.

Except 75 , all other numbers have two factors.

115 = 5 x 23

 95  = 5 x 19

 85  = 5 x 17

 75  = 3 x 5 x 5