Step 1: Compare crust types.
Oceanic crust is thin, about 5 to 10 km, while continental crust is much thicker.
Step 2: Add the mountain effect.
Under tall mountains the crust grows a deep root downward to support the great height above.
Step 3: Apply to the Himalayas.
The Himalaya is the tallest range on land, so its crust must be the thickest.
Step 4: Use the known value.
Beneath the Himalayan region the continental crust reaches about 70 km in thickness.
Step 5: Check the options.
Values like 5, 15 and 30 km are far too small for such a high range, leaving 70 km.
Step 6: Conclude.
The continental crust under the Himalaya is about 70 km thick.
\[ \boxed{70\ \text{km}} \]