Answer:
Rhombus
Step-by-step explanation:
So if |AB| = |BC| then ABCD is a rhombus and has a second line of reflective symmetry, namely \overleftrightarrow{BD}. Quadrilateral ABCD is what is called a kite.
Since ℓ contains two vertices of ABCD the other two must be reflections of one another about ℓ. In the picture ℓ goes through A and C and D=rℓ(B). So rℓ interchanges AD¯¯¯¯¯¯¯¯ and AB¯¯¯¯¯¯¯¯ and also interchanges CB¯¯¯¯¯¯¯¯ and CD¯¯¯¯¯¯¯¯. So if |AB|=|BC| then ABCD is a rhombus and has a second line of reflective symmetry, namely BD←→. Quadrilateral ABCD is what is called a kite. Note that we have implicitly used the fact that ABCD is convex: without this hypothesis, the vertex C could be reflected over BD←→, giving a non convex quadrilateral with exactly one line of symmetry.
