(C) Infinitely many
Let the linear equation be ax + by + c = 0.
On putting x = 1 and y = 2, in above equation we get
s = a + 2b + c = 0, where a, b and c, are real number
Here, different values of a, b and c satisfy a + 2b + c = 0. Hence, infinitely many linear equations in x and y be satisfied by x = 1 and y = 2.
