//in class A:
public class A ...

private B b; // B is the classname of the object you called B.

public void act() {
    if (b == null) {
        if (getWorld().getObject(B.class).size() != 0) {
            b = (B) getWorld().getObjects(B.class).get(0); // this gives the reference to the first object from type B the method getObjects has found.
            
        }
    }
    else if (Greenfoot.mouseKlicked(this)) {
        b.methodYouWantToExecute();
    }
}

import greenfoot.*;

public class Unarmed extends Armed
{
    public void act() // to over-ride 'act' in 'Armed'
    {
    }
    
    public void triger() // to over-ride 'triger' in 'Armed'
    {
    }
}

public Armed() // for use by 'Unarmed'
{
}

public Armed(House explodingHouse) // for use by your sub-class of World
{
    thatHouse = explodingHouse;
}

House house = new House();
addObject(house, 311, 258);
Armed armed = new Armed(house); // notice the parameter
addObject(armed, 78, 421);

import greenfoot.*;

public class Plunger extends Actor
{
    GreenfootImage plungedImage = new GreenfootImage("explosivePlungerUnarmed.png");
    boolean armed = true;
    House explodingHouse = null;
    
    public Plunger(House inHouse)
    {
        explodingHouse = inHouse;
    }

    public void act() 
    {
        if(armed && Greenfoot.mousePressed(this))
        {
            setImage(plungedImage);
            setLocation(getX(), getY() + 5);
            armed = false;
            explodingHouse.explode(); 
        }
    }    
}