He amount of current flowing in a circuit can be measured by
a. an ammeter.
b. a voltmeter.
c. a neon test lamp.
d. an ohmmeter.
user:
3. which one of the following letters is used in formulas as the symbol for voltage?
a. p
b. w
c. i
d. e

First question:
The magnitude of current flowing in a circuit is described in
units of Amperes.  The device used to measure it is an Ampmeter,
or Ammeter.

Second question:
This question is so absurd that it should not be dignified with an
answer.  Although 'E' is often used as the symbol for Electromotive
force, potential difference, and voltage, there's certainly no rule. 
Anyone is free to use  'M',  'Q',  'Θ', or  'Щ' to denote voltage when
they write electrical formulas, just as long as they make sure to
explain the meaning of whatever symbols they use.