Sensor communicating with a base station 3.5km away for a rate of 100bytes every 10 sec
315 MHz is probably your best option given that you'll get longer range compared to higher frequencies. This is because an antenna at lower frequencies has (what is called) a wider "aperture". It works like this....

A transmit antenna (dipole or monopole) generally sprays the RF power out 360 degrees in all directions horizontally and also in a fairly wide arc vertically. Because of this, RF engineers talk about the EM power flowing thru one square metre of area at a certain distance from the TX antenna - the bigger the distance from the TX antenna, the smaller the power per sq metre because the area-coverage is bigger.

Power per sq metre falls as distance squared in simple terms.

At lower frequencies, the receive antenna's effective aperture (also measured in sq metres) is bigger because the antenna dipole length is proportional to wavelength. A longer resonant antenna means a bigger aperture and that means more power received.

The effective antenna area "captures" all the picowatts per square metre flying thru its aperture hence, at a lower frequency a dipole receives more power.

I hope that justifies why 315 MHz is probably the best frequency to go for. However, if there was an ISM frequency at (say) 1MHz this might not be a good frequency but this is based on a terrain/line-of-sight analysis. Whole books can be written on this!

Moving on....

You might have problems staying within legal power limits AND getting 3.5km. It's not a problem for a bespoke radio that is designed for slow speed data comms but, who on earth is going to design an off-the-shelf module that has a data rate limited to 10 bytes per second. Having said that, in the dim and distant past (probably 1980s) I remember one telemetry radio designer say that their output power was 1uW and it could be recieved by a suitable receiver 1kM away - bandwidth was really low - maybe 10 Hz (from memory)

Slow data rates mean the receiver sensitity can be massively improved because it can have a much tighter RF filter that blocks out a lot more "ambient" noise than a faster receiver. With a low bandwidth receiver you can easily get several km on 1mW but finding an off-the-shelf one is going to be a misery.

If you can avoid the uplink sample rate control back to the field transmitter you are more-than halving the problem re battery power and complexity - find a way of tackling this first is my advice.