An instrument which can measure terminal potential difference, as well as electromotive force (e.m.f.), is:
(A) Wheatstone's meter bridge
(B) Voltmeter
(C) Potentiometer
(D) Galvanometer

Hint: The given problem can be solved using the concept of the cell when it is in an open circuit (i.e. when no load is connected to the cell) and when the cell is in a closed circuit (i.e. when the load is connected to the cell).

Complete step by step answer:
Voltmeter is used to measure both terminal potential difference as well as electromotive force (e.m.f.) depending upon the connection state of the battery or cell.
The terminal potential difference and electromotive force (e.m.f.) can be defined as an effect and a cause of the given circuit respectively in which battery or cell is connected.
The terms given like an effect to the terminal potential difference and a cause to electromotive force (e.m.f.) can be understood as given below-

Electromotive force (e.m.f.): Electromotive force (e.m.f.) of any cell can be defined as the maximum potential difference between the two electrodes of the cell when the cell is in the open circuit and here cell is in an open circuit means when no current is drawn from the cell.
It is generally denoted by \[\varepsilon \] and is measured in volt.
The electromotive force (e.m.f.) of a cell is independent of
The quantity of electrolyte that is used in the cell
The size of the electrodes
The distance between the electrodes
The electromotive force(e.m.f.) is used for the source of the electric current for any given circuit in which the cell is going to be used.

Terminal potential difference: Terminal potential difference of a cell is defined as the potential difference between the two electrodes of a cell in a closed circuit and here closed circuit means when current is drawn from the cell.
It is generally denoted by $V$ and is measured in volt.
The terminal potential difference of a cell depends upon
The resistance between the two points of the circuit
The current flowing through the circuit
The terminal potential difference is measured between any two points of the electric circuit.
The terminal potential difference between the two electrodes of the cell becomes less than the e.m.f. of the cell by an amount equal to the potential drop across the internal resistance of the cell.

Note:
While measuring these two quantities, it is important to remember that the difference of e.m.f. and terminal voltage is called lost voltage as it is not indicated by the voltmeter.
This lost voltage can be defined as \[Ir\] where \[I = \]current drawn by the cell and \[r = \] internal resistance of the cell.
The direction of current sent by a cell in an electric circuit is from a positive terminal to a negative terminal outside of the cell and negative terminal to the positive terminal inside the cell.