Correct spelling, explanation: sustain is the correct form as the word comes from Latin sustinere, which is also spelled without b, as it is in an incorrect option: substain. Therefore the correct spelling is sustain.

Definition of sustain:
verb, to allow something to continue
Right now the doctors sustain her life with special equipment, so we don’t know if she will make it.
You should drink a lot of water to sustain your good health.

Incorrect spelling, explanation: substain is a wrong spelling, probably mistaken with other words starting with prefix sub-. The correct spelling is sustain as the word derives from Latin sustinere. Sustain is the only correct form of this verb.

Incorrect spelling

Incorrect spelling