These series of attacks on Vietnam are called the Tet Offensive.
Let's take a look at each answer individually.
The Fall of Saigon - this was specifically one place in the South, so it couldn't have been a series of attacks throughout the country. Here's a fact: the Fall of Saigon was what led Vietnam to fall under communist rule since Saigon was considered the capital and main point for the South (which the U.S. helped).
The My Lai massacre was also another single attack made on My Lai (in the middle of Vietnam). Again, this was specifically one attack on one village, so it wasn't a major scale attack, but it was a major discussion point and an important historical event for the Vietnamese.
Let's talk about the Battle of Da Nang now - again, this was one battle, it wasn't a series of battles across the country, therefore it isn't the answer. This battle was fought in the middle of the country (in Vietnam) for the main control of fronts and the main cities - Hue and Da Nang, often considered MSRs to some military experts.
Finally, on the Tet Offensive - this was basically a "push" to capture major points and included several attacks on the South's main control points. This ended with the U.S. pulling back to avoid war and further criticism from the American people, and southern Vietnam losing the country to the communists.
