Technically all of our applications are accessible from mobile devices, but each has varying degrees of responsiveness.
Treejack is mobile-optimized, meaning that it works well on a mobile device. There should be no problem participating in a Treejack study from a mobile device, although we always recommend testing out the study on the intended device(s) prior to launch.
OptimalSort studies are accessible however, the size of the sorting canvas can get pretty cramped very quickly if you have a large number of items to sort, resulting in a less-than-ideal experience for participants. Tablets are by far preferable to a smartphone due to size, but it is still something that you should test out thoroughly so that you can decide whether or not the size of the sort is conducive to the limitations on the size of the screen to prevent any frustration on the part of your potential participants.
Chalkmark will display an image at the same size and resolution as the original image that is uploaded. So if you have a study intended for display on a larger monitor, it won't work well on a mobile device. However, you can set up a Chalkmark study specifically designed for a mobile device and upload an appropriately sized image and selecting the "Scale on smaller screens" option on the Tasks tab.
Questions is a very new tool but again, is mobile compatible. The experience is just reduced on a smaller device, but the participant can successfully complete their study without issue.
No matter the tool or the study, we would strongly recommend adding information in the welcome screen or in a pre-study questionnaire to let them know the intended device(s) they should be using if they choose to continue, as well as perhaps trying to ask participants what exact device they are using so you can factor that into the data analysis.