Why Voltage is a difference in potential between two points, so a voltmeter must touch both ends of the component at once, which means connecting it across (in parallel with) the part. A voltmeter has very high internal resistance, so bridging it across a circuit draws almost no current and barely disturbs what you are measuring. Ammeters are the opposite: low resistance, inserted in series so all the current flows through them.
Watch out The series choice describes how an ammeter is connected; wiring a high-resistance voltmeter in series would block current and give a meaningless reading. Quadrature and in phase refer to signal phase relationships, not meter hookups.
Volts across, amps through. Voltmeter goes in parallel, ammeter breaks the circuit.
PARALLEL has two L's standing side by side, like two probes touching both ends of a part at once. Voltmeter = parallel.