$20,000 at 40% gives you $8,000 of return from that part of the money - this is found via simple multiplication, as there is no time period given in the question as listed, so I'm assuming simple interest for a single year.
25% of the total of $100,000 to invest is $25,000.
Since the total is $25,000, and the first part gives you $8,000, subtract the $8,000 from the $25,000 to find out how much more you need: $17,000.
$17,000÷$80,000 = .2125, or 21.25%. This rounds to answer c), 21.3%.
The other way to approach this is to treat it as a "mixture" problem, as in the algebras (prealgebra+, depending on what state and or school you were in and when you learned it; these are occasionally also seen in "grade level" math classes). A + B = 100,000, and (rate of return for A) *A + (rate of return for B) * B = total return; total return divided by the total invested is the average rate of return. The math comes out the same either way.
If this were an in-person tutoring session, I would want to flip through the relevant section of the textbook just to make sure they aren't expecting something else, like treating it as continuously compounding. Some problem sets have an introductory statement saying "do it this way" or "make these assumptions". As it stands, there's a slightly ominous feeling of "that should have been harder - what am I missing", but with just the freestanding question, this is my best answer for what I believe they're expecting and why.
