If heat is supplied to a solid, it temperature

i) must increase

ii) may increase 

iii)may remain constant

iv) may decrease

ii) may increase because the heat energy supplied vibrates the atoms of the solid faster about their mean position, thus increasing the energy and hence the temperature of the substance.

iii)may remain constant because the heat energy supplied is used in increasing the inter-atomic distances between the atoms of the solid, due to which they are no longer tightly bound to each other and are held loosely bound, resulting in a change of state (solid to liquid or gas).