Crypto Algorithm Question - Exponential Cipher

There is 26-letter English alphabet. There is the plain text: HELLO
Encode the text with exponential cipher (Ek(x) = 7x3 mod(26)).
The cipher text is:
a. not possible to encode , it is against the rules
b. not possible because there is not enough information
c. JGJJU
d. MZDDP

One of those is quite obviously the answer, without doing any working out. Look at the pattern of the plaintext, then look at the pattern of the ciphertext.