We have that
triangle ABC coordinates
A (0,2) B(2,4) C(0,0)

triangle DEF coordinates
D (2,0) E ( 4,4) F (4,2)

using a graph tool
see the attached figure

therefore

Line 1 Segment AC equals 2. Segment FE equals 2. Segment AC is congruent to segment FE
so
Segment AC equals 2-------> is correct
Segment FE equals 2-----> is correct
Segment AC is congruent to segment FE------> is correct

Line 2 ∠A ≅ ∠F------> is correct

Line 3 Length of segment AB. A (0, 2) B (2, 4) d equals square root of quantity x sub 2 minus x sub 1 squared plus quantity y sub 2 minus y sub 1 squared, d equals square root of quantity 0 minus 2 all squared plus quantity 2 minus 4 all squared, d equals square root of negative 2 squared plus negative 2 squared, d equals square root of 4 plus 4, d equals square root of 8 segment AB = 2.83

find the distance AB
d=√[(4-2)²+(2-0)²]--------> d=√[4+4]-----> d=√8-----> d=2.83
AB=2.83
so
Line 3 is correct

Line 4 Length of segment DE. D (2, 0) E (4, 4) d equals square root of quantity x sub 2 minus x sub 1 squared plus quantity y sub 2 minus y sub squared, d equals square root of quantity 2 minus 4 all squared plus quantity 0 minus 4 all squared, d equals square root of negative 2 squared plus negative 4 squared, d equals square root of 4 plus 16, then d equals square root of 20 segment DE = 4.47

find the distance DE
d=√[(4-0)²+(4-2)²]--------> d=√[16+4]-----> d=√20-----> d=4.47
DE=4.47
so
Line 4------> is correct

Line 5 segment AB is congruent with segment DE
AB is not congruent with DE
so
Line 5 is not correct

therefore

the answer is
the student make the first mistake in line 5