I wouldn't use 3, but the others seem okay.
To me, here's what the prepositions mean:
against -- next to
on and onto -- on top of
1. Raise your right foot and place it against your left thigh.
2. Raise your right foot and place it on your left thigh.
3. Raise your right foot and place it to your left thigh.
4, Raise your right foot and place it onto your left thigh.
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Can we use other prepositions instead of 'against'? Which is grammatical? Do they have any difference because of the diference of the preposition in meaning?
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