The sun was _______ boiling this afternoon.

- The correct answer is Absolutely.  it is an intensifier. 

- Really and very are also intensifire but these two indicates general meaning or a lot. But by absolutely it means excessively or extremely. 

Intensifiers:

These are adverbs or adverbial phrases that strengthen the meaning of other expressions and show emphasis.

Words that we commonly use as intensifiers include:

- absolutely
- completely
- extremely
- highly
- rather
- really
- so
- too
- totally
- utterly
- very

Source: Cambridge dictionary.com