The Seebeck effect is the phenomenon in which an [latex]emf[/latex] (called thermo-emf) is generated in a closed loop made of two dissimilar metals when their two junctions are kept at different temperatures. This thermo-emf, [latex]\varepsilon[/latex], depends on the temperature difference between the hot and cold junctions and drives a current through the circuit, known as a thermoelectric current.

For quick reference, here's why the other options don't fit:

b. Joule effect — This describes the heating produced in a conductor due to the flow of current, given by [latex]H = I^2Rt[/latex]. It doesn't involve producing emf from a temperature difference.
c. Peltier effect — This is essentially the reverse of the Seebeck effect: when a current is passed through a junction of two dissimilar metals, heat is either absorbed or released at the junction. Here, current causes a temperature change, not the other way around.
d. Thomson effect — This refers to the absorption or evolution of heat when current flows through a single conductor that has a temperature gradient along its length, not across a junction of two different metals.

Since the question specifically asks about emf being produced due to a temperature difference between two junctions of dissimilar metals, the correct answer is (a) Seebeck effect.

The Peltier effect is the production or absorption of heat at a junction of two dissimilar metals when current is passed through it — one junction heats up while the other cools down. It is the reverse of the Seebeck effect, where a temperature difference produces an [latex]emf[/latex] instead of current producing a temperature change.

Temperature difference between two junctions of dissimilar metals producing an [latex]emf[/latex] is the definition of the Seebeck effect.

Using [latex]t_n = \dfrac{t_i + t_c}{2}[/latex], we get [latex]t_c = 2t_n - t_i = 2(310) - 600 = 20^\circ C[/latex].

In a thermoelectric series, emf increases with the separation between the two metals in the list — metals closer together give less emf. Zn and Mn are adjacent in the given series, so they produce the least emf among the options.

emf produced due to a temperature difference between two junctions of dissimilar metals is the Seebeck effect.

At neutral temperature, emf is maximum, so [latex]\dfrac{dE}{d\theta} = 0[/latex]. Differentiating: [latex]a + b\theta_n = 0 \Rightarrow \theta_n = -\dfrac{a}{b}[/latex].