Here is the item:
While Karen was cleaning green peppers at the sink, Chris __________ a long, fat anchovy onto the pizza, camouflaging it with sliced mushrooms and onions.

Chris put the anchovy on the pizza. Thus, you need a form of lay. Since the sentence is past tense, you need the simple past form.

You chose the correct form from the table below. Good job!

Infinitive Definition Simple
Present
Simple
Past
Past
Participle
Present
Participle
to lay to put something down lay(s) laid laid laying
to lie to rest or recline lie(s) lay lain lying

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