Final answer:
In Android development, the attribute to change a linear layout to horizontal orientation is 'android:orientation="horizontal"'.
Step-by-step explanation:
The attribute you can add to switch a linear layout to horizontal orientation in Android development is b. android:orientation="horizontal". The android:orientation attribute specifies the orientation of a LinearLayout. The default orientation is vertical. By setting this attribute to "horizontal", the layout will arrange its children in a row, side by side. Neither android:weight nor android:gravity nor android:layout can change the orientation of the linear layout to horizontal.