(a) 1 : 2
Thermal energy developed across a resistor,
U = i2Rt ,
where i is the current flowing through the resistor of resistance R for time t. Since the resistors are connected in series, the current flowing through both the resistors is same and the time for which the current flows is also same.
Thus, the ratio of the thermal energy developed in R and 2 R is 1 : 2.