Which of the following triangles are acute triangles?

For triangle I, all the internal angles of this triangle are <90^{°}. Hence, it is an acute triangle.

As for triangle II, one of the internal angle is 90°. Since this angle is not <90^{°}, it is not an acute triangle.

Now, for triangle III, one of the internal angle is 111° which is not <90^{°}. Hence, it is not an acute triangle.

For triangle IV, all the internal angles are <90^{°}. Hence, it is an acute triangle.

Clearly, only triangle I and IV are acute triangles. Therefore, the answer is D.

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