Answer:
a)True
b)False
c)False
Step-by-step explanation:
We know that ideal diesel cycle is the ideal cycle for all practically used diesel cycle. Ideal cycle do not have any internal or internal ir-reversibility ,all the processes are reversible process.
In diesel engine there are four process
Process 1-2:Reversible adiabatic compression process
Process 2-3:Constant pressure heat addition
Process 3-4-:Reversible adiabatic expansion process
Process 4-1:Constant volume heat rejection
Efficiency of diesel engine given as follows

So it is clear that Efficiency is not only a function of compression ratio(r) only.
When maximum temperature will increases then cut of ratio(ρ)also will increases and this leads to decrease in the efficiency of cycle.
So
a)True
b)False
c)False