An equation is said to be linear equation in two variables
if it is written in the form of ax + by = c, where a, b & c 
are real numbers and the coefficients of x and y, i.e 
a and b respectively, are not equal to zero.

a.     
b.    
c.     
d. 

In the choice (b.),  there is no y in the first equation,
which means that -x = 5 is equivalent to -x + 0y = 5, which
means that the coefficient of y is equal to 0.  That means
that it cannot be a linear equation in two variables.

Also in the choice (b.), there in no x in the second equation,
which means that y - 4y = 2, or -3y = 2 is equivalent to
0x - 3y = 2, which means that the coefficient of x is equal
to 0.  That is further reason that it cannot be a linear
equation in two variables.

Edwin