"Sonnet XXXI"
With how sad steps, Oh Moon, thou climb'st the skies!
How silently, and with how wan a face!
What, may it be that even in heavenly place
That busy archer his sharp arrows tries?
Sure, if that long-with-love-acquainted eyes 5
Can judge of love, thou feel'st a lover's case,
I read it in thy looks; thy languished grace,
To me, that feel the like, thy state descries.
Then, even of fellowship, Oh Moon, tell me,
Is constant love deemed there but want of wit? 10
Are beauties there as proud as here they be?
Do they above love to be loved and yet
Those lovers scorn whom that love doth possess?
Do they call virtue there ungratefulness?

—Sir Philip Sidney

Which line contains an allusion?

a. line 1
b. line 4
c. line 9
d. line 11

In Sonnet XXX or Sonnet 31, Philip Sydney (1554 – 1586) comments on the pale and sad appearance of moon, and he wonders why it is so. He asks the sun in line 4 whether there is also love which makes him pale and sad. He asks this by alluding to Cupid (the Roman god of love). Cupid is often portrayed with a bow and an arrow, and whoever is shot by that arrow falls in love. So, whoever falls in love is figuratively said to have been shot by Cupid's arrow.

The line 4 alludes to Cupid by calling him "that busy archer". Apart from this mythical allusion there is no allusion in this sonnet.