It is wrong because B sharp is a major seventh of C# and there should be a minor seventh in a 7th chord.

What is your question exactly?

Major 2 is not the second note in the c major scale like you thought. 

Perfect prime = the same note ex. P1 of c = c

From low to high is sharp. From high to low is flat

As you can see there is no black key between E and F and thus there's only 1 semitone between the notes. This makes the interval a minor second (which you will learn about at a later stage in the app).

Try reading more carefully in the wiki or watch the video again, it clearly states that:
a major second is an interval from a natural to the next one above it, provided that there are also 2 semitones between the notes.

It is not a bug because a capital M means major, lowercase m means minor…

You’ve played the note F…

Most likely you did not need to play A3, but as you haven't input any text as a question it's hard to know what you don't understand.

B to C flat is not a major second but a diminished second. It’s showing you the correct answer (B to C sharp).

F is not E#, they only sound the same. It’s because you can higher and lower naturals with flats and sharps.

So E sharp and F sound the same, but so do:
E and Fb
G# and Ab
D## and E
A# and Cbb
Although double sharp or flat notes are rare, they do exist.

When to use F or E sharp depends on the scale or chord or key you want to play.

If you look at the piano you can see that there's a repetitive pattern of 2 and 3 black keys. The note C is always the first white key on the left of the 2 black keys. So when you see three black keys you must be seeing the white keys F G A B.

Hope that helps!

As answered on your other similar question:

F is not E#, they only sound the same. It’s because you can higher and lower naturals with flats and sharps.

So E sharp and F sound the same, but so do:
E and Fb
G# and Ab
D## and E
A# and Cbb
Although double sharp or flat notes are rare, they do exist.

When to use F or E sharp depends on the scale or chord or key you want to play.

Because it says you heard a minor 2nd

Thank you!