https://youtu.be/Xcln3xG8QGQ
Refer the video to understand algebraic multiplicity. And it is defined for each distinct eigen value. The characteristic equation is given by (a - 1)^3 = 0.
So it has only one distinct eigen value and it has algebraic multiplicity of 3 bcoz of term (a-1)^3.
Option C) 3 is the answer