The correct answer is A) the notation for the imaginary unit, C) the formalization of function notation, and E) the notation for the base of the natural logarithm.
The mathematical concepts that Euler's legacy includes are the notation for the imaginary unit, the formalization of function notation, and the notation for the base of the natural logarithm.
Leonhard Euler (1707-1783) was a Swiss mathematician that developed important concepts in differential equations, geometry, calculus, and trigonometry that were applied in different fields such as mechanics, astronomy and acoustics. His concepts also had a deep effect on engineering.