The sentence "Jessica can't be in Italy because I saw her in the supermarket today" expresses negative deduction.
The speaker concludes that Jessica cannot be in Italy based on the fact that they saw her in the supermarket on the same day. Negative deduction involves drawing a logical inference by ruling out a possibility based on contradictory evidence or information. In this case, the observation of Jessica in the supermarket directly contradicts the possibility of her being in Italy. Therefore, the sentence expresses negative deduction.