Trying to find transistor needed for laser security alarm
If your LDR has an 'ON' resistance of 100 kOhms, that means the most current it can shunt away from the base of your transistor is say 0.4V/100k = 4 uA. So, the converse of this is that when the LDR is 'OFF', you only have a 4 uA base drive (if it were more it would overdrive the LDR and switch on the npn all the time). Which means if you use a transistor with a \$\beta=200\$ the most current it will sink is 800 uA.

You will want to know what your LED and buzzer operating current is.

Most likely you will want a FET or a darlington with no base-emitter resistor shunts.