Count four beats

In 4/4, the top 4 means four beats in a bar. The bottom 4 makes the quarter note one beat. A filled notehead with a stem is a quarter note: 1 beat. An open notehead with a stem is a half note: 2 beats. A rest is silent, but still takes time.

This bar has a quarter note, a half note and another quarter note. Their lengths are 1 + 2 + 1 = 4 beats. Three symbols can fill four beats. Count their durations rather than counting the number of notes.

The two quarter notes already use 1 + 1 = 2 beats. A 4/4 bar needs four. Work out how much time is left, then choose the note that lasts that long.