Which of the following compounds is most acidic?

In the periodic table,metallic character of the elements shows which one of the following trends?

$Be$ and $Al$ show similarities in properties due to the diagonal relationship except in the property $X$ given below. What is $X?$

The acidic,basic and amphoteric oxides,respectively,are

Considering the elements $B$,$Al$,$Mg$,and $K$,the correct order of their metallic character is:
$(a)$ $B > Al > Mg > K$
$(b)$ $Al > Mg > B > K$
$(c)$ $Mg > Al > K > B$
$(d)$ $K > Mg > Al > B$

Which of the following is the correct order of acidic strength?