The correct answer is: D. instantaneous value
The instantaneous value of flux is the value of the flux at a particular instant in time. It is denoted by $\phi(t)$. The average value of flux is the average of the instantaneous values of flux over a period of time. It is denoted by $\bar{\phi}$. The r.m.s. value of flux is the square root of the mean of the squares of the instantaneous values of flux over a period of time. It is denoted by $\phi_{rms}$. The maximum value of flux is the largest value of the instantaneous values of flux over a period of time. It is denoted by $\phi_{max}$.
In the e.m.f. equation of a transformer, the value of flux involved is the instantaneous value of flux. This is because the e.m.f. induced in the secondary winding of a transformer is proportional to the rate of change of the flux in the primary winding. The rate of change of flux is the derivative of the flux with respect to time, which is the instantaneous value of flux.
The average value of flux is not involved in the e.m.f. equation of a transformer because the average value of flux does not change with time. The r.m.s. value of flux is also not involved in the e.m.f. equation of a transformer because the r.m.s. value of flux is not proportional to the rate of change of flux. The maximum value of flux is also not involved in the e.m.f. equation of a transformer because the maximum value of flux is not always the same as the instantaneous value of flux.