The correct answer is $\boxed{\text{A}}$.
The force in a member of a truss can be calculated using the following formula:
$$F = \frac{P}{L}$$
where $F$ is the force in the member, $P$ is the load on the truss, and $L$ is the length of the member.
In the truss shown in the question, the load on the truss is $P = 12.5 \text{ t}$ and the length of member AC is $L = 2 \text{ m}$. Therefore, the force in member AC is:
$$F = \frac{P}{L} = \frac{12.5 \text{ t}}{2 \text{ m}} = 6.25 \text{ t}$$
The force in member AC is compressive because the member is in compression. This can be seen by the fact that the member is being pushed together by the other members of the truss.
The other options are incorrect because they do not represent the correct force in member AC. Option B is incorrect because the force in member AC is not 8.75 t tensile. Option C is incorrect because the force in member AC is not t tensile. Option D is incorrect because the force in member AC is not t compressive.