Matrix A is a matrix of order 2.
Identity matrix of second order is [

1	0
0	1

]
For A to be an identity matrix,
[

1	0
0	1

] = [

cosα	−sinα
sinα	cosα

]
⇒ cos α = 1 and sin α = 0
⇒ cos α = cos 0° and sin σ = sin 0°
⇒ α = 0°
Thus, for α = 0° , A is an identity matrix