How do I hide the objects for some time ?

Dear! What I have to do is to remove on object on some event and appears another at the same time in the same place , for example, there are four objects on the screen Apple, Ball, Cat and Dog, Now when user Drag "A" to the Apple which is the right answer, Apple + A both removes which i did and "B" appears at the same position where was "A" in the start, and another object like "Monkey" appear where was "Apple" and same from A to Z.

screen shot is attached.

In image both A and B appears but I want to show B when A is removed.

You can switch objects from visible to invisible and back try that in collaboration with a Global Variable

I tried by setting as Visible and Invisible , but Once it becomes invisible then never appears untill I no chnage the propertyto visible,

If you are working with sprites, you can play with the frames. For example, you can create one sprite for the letters and another for the animals, with the same frame order. Then, when you need to change both images you just have to change to the next frame (using a global variable that specifies which frame are you in) and you won't need to play with its visibility.

You could use list-box like this tutorial

https://www.scirra.com/tutorials/879/listbox-tutorial