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We started a photometric survey using the WFC3/UVIS instrument onboard theHubble Space Telescope to search for multiple populations within MagellanicCloud star clusters at various ages. In this paper, we introduce this survey.As first results of this programme, we also present multi-band photometricobservations of NGC 121 in different filters taken with the WFC3/UVIS andACS/WFC instruments. We analyze the colour-magnitude diagram (CMD) of NGC 121,which is the only "classical" globular cluster within the Small MagellanicCloud. Thereby, we use the pseudo-colourC_(F336W,F438W,F343N)=(F336W-F438W)-(F438W-F343N) to separate populations withdifferent C and N abundances. We show that the red giant branch splits up intwo distinct populations when using this colour combination. NGC 121 thusappears to be similar to Galactic globular clusters in hosting multiplepopulations. The fraction of enriched stars (N rich, C poor) in NGC 121 isabout 32% +/- 3%, which is lower than the median fraction found in Milky Wayglobular clusters. The enriched population seems to be more centrallyconcentrated compared to the primordial one. These results are consistent withthe recent results by Dalessandro et al. (2016). The morphology of theHorizontal Branch in a CMD using the optical filters F555W and F814W is bestproduced by a population with a spread in Helium of Delta(Y) =0.025+/-0.005.

The Search for Multiple Populations in Magellanic Cloud Clusters I: Two Stellar Populations in the Small Magellanic Cloud Globular Cluster NGC 121