Which programming languages used to make sololearn apps ..

Answer 1

Java, XML, and the Android development kit.

Answer 2

@mithun %d is only a c thing? nope sry....

Answer 3

kotlin and java 100% ✓

Answer 4

* major portion of sololearn written in C language..

you can see at the time of challenge
under your avatar * level %d *..
%d is written in C..
so proved..