What is the value of sin a for this triangle?

The sine of the angle is the ratio of the opposite side to the hypotenuse. The hypotenuse of this triangle can be computed from the Pythagorean theorem:
.. AB = √(AC^2 +BC^2) = √(9 +4) = √13 . . . . cm
Then the ratio of opposite side BC to the hypotenuse AB is
.. sin(A) = BC/AB = 2/√13

Corresponds to the 3rd selection.

B-simple yes/no questions