According to Masters and Johnson, a woman experiencing a leveling off of high arousal, a tightening of the vaginal opening, an expansion of the upper third of the vagina, and increasing sexual tension is in the phase of sexual arousal.

The answer is plateau phase of sexual arousal.

Explanation:

The plateau phase of sexual arousal is the point of sexual activity which a woman starts experiencing increased sexual excitement before orgasm (climax).  

Asides a leveling-off  (a steady level) of sexual arousal, other bodily responses occur during the plateau phase. The bodily responses - according to Masters and Johnson - include expansion of the upper third of the vagina, tightening of the vaginal opening, and increased sexual tension.

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