The singular form in stimulus.
it is already in its singular form.
The form hypothesis is the singular form. The plural form is hypotheses.
The singular is cavity.
The singular form is phenomenon.
Criterion
The plural for stimulus is stimuli.
stimulus = singlestimuli = plural
Stimulus can be both singular and plural depending on its usage. As a singular noun, it refers to a single influence or prompt. As a plural noun, stimuli is used to refer to multiple influences or prompts.
plural
"Stimulus" is singular; "stimuli" is plural.
Stimulus (singular) and Stimuli (plural)
Because "stimulus" is a Latin word. In Latin, most words ending in "-us" form the plural in "i". They keep this in English.
The noun stimulus is singular. The plural is stimuli - as in "The patient did not respond, even to repeated stimuli". Note that 'stimulus' is becoming increasingly used as an adjective - stimulus measures, a stimulus package - since the credit crunch of 2009.
The singular form of the demonstrative pronoun 'these' is this.
The singular form of feet is foot.
Stimuli is the plural form of "stimulus," which means something that is intended to enact a reaction in an organism.
The singular form of "hypothysis" is "hypothesis."