Which choice will decrease the size of the effective focal spot?

Decreasing the target angle will effectively reduce the size of the effective focal spot. The effective focal spot is the area from which the X-rays are emitted, and it is influenced by the target angle of the anode in the X-ray tube. When the target angle is decreased, the geometric projection of the focal spot onto the image receptor becomes smaller, resulting in a finer focal spot. This enhances image sharpness and spatial resolution, which is particularly important in radiography for capturing detailed images of structures.

In contrast, increasing the target angle would enlarge the effective focal spot, as a larger angle creates a broader area for X-rays to be emitted. Adjusting the technique or exposure time does not specifically alter the size of the effective focal spot; instead, they influence other aspects of image quality, such as exposure and contrast. Therefore, the most effective method for decreasing the size of the effective focal spot is by decreasing the target angle.