The angle between A= î+ĵ and B= î-ĵ is

The correct option is 90°.

To find the angle between two vectors $A$ and $B$, we can use the definition of the dot product (scalar product):
$A·B=\left|A\right|\left|B\right|cos\left(\theta \right)$
where $\theta$ is the angle between the two vectors.

Given vectors:
$A=i+j$
$B=i-j$

First, let's calculate the dot product of $A$ and $B$:
$A·B=\left(1\right)\left(1\right)+\left(1\right)(-1)$
$A·B=1-1=0$

Since the dot product of the two non-zero vectors is zero, the vectors are orthogonal (perpendicular) to each other.

Mathematically:
$0=\left|A\right|\left|B\right|cos\left(\theta \right)$
Since $\left|A\right|\ne 0$ and $\left|B\right|\ne 0$, we must have:
$cos\left(\theta \right)=0$
$\theta =90°$

Therefore, the angle between the two vectors is 90°.