Calculate the magnetic field strength?

Length of coil = 0.96 m

Amp turns (At for short) = 6.24 x 390 = 2434

Therefore magnetic field strength H in At/m = 2434/0.96 = 2535.

Hence option 1 is the answer.

Edit: AZ: The asker correctly and clearly requests the magnetic field strength in ampère turns/m (At/m) for which the standard SI symbol is H.
We may obtain the flux density B in Tesla (free space value) from the simple relationship B=μ₀.H (to which you effectively refer) where μ₀ = 4π×10⁻⁷. Admittedly in recent years I do quite commonly see the flux density referred to (I believe originally erroneously) as magnetic field strength but the origin of this loose and confusing usage is unknown to me. Anyhow the H value is independent of μ, although the μ₀ value will be very close for mica. So μ is irrelevant here as indeed is the cross sectional area.


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Sounds like what is described is a toroid.
B = mu * N * I / (2*pi*R).
I have no idea what the "mu" value for mica would be (its magnetic permeability), but mica is a good insulator like air, so maybe you just ignore the mica and use mu-nought (4*pi x 10^7 H/m). N = 390 and 2*pi*R = 96 cm = 0.96m.
The units "At/m" are not familiar to SI users. Must be something used in your country...